It’s obvious that Lizzie and Mr. Collins would never have made a match. But why didn’t Mrs. Bennet steer him towards Mary? Yes, she’s a younger daughter, but it would keep the estate in the family and the two of them could be pompous and insufferable together, and would probably be as happy as either could be.
StG
Because it wouldn’t have suited the plot, silly. 
I always thought that Mary and Mr. Collins would have been a good match. Maybe she was too young? Lydia was sixteen when she ran off with Wickham, right? If Mary was the middle daughter, she could have been only 17 when Mr. Collins came on the scene. Seventeen might have been too young for a clergyman’s wife.
Brynda - True. A much shorter novel, to be sure.
burundi - Lizzie was not yet 21. Jane was the oldest, then Lizzie, Mary, Kitty and Lydia. So Mary was probably about 19 and only just a bit younger than Lizzie, who was definitely marriagable.
StG
One of the most important plot points is that Mrs. Bennet was a supremely silly woman and Mr. Bennet profoundly inattentive.
Remember that Mr. Collins was working his way down the list in order of seniority; he wanted to propose for the eldest, but was informed a match was assured for her with … that silly git whose name I forget.
Any decent set of parents who had five daughters of small portion to marry off would have been nudging him toward Mary before the first dinner was over.
Mr Collins had made the “polite” offer to the Bennets for one of their daughters. Once both Jane and Elizabeth were out of the running perhaps Mary would have seemed a reasonable alternative - if not for the presence of Charlotte Lucas, a more sensible woman who was champing at the bit for a husband. Mr Collins wasn’t a complete fool.
Mr Charles Bingley
Well, and I think that although Mary quite liked Mr. Collins, Mr. Collins was looking for someone a bit more attractive than Mary. I know Charlotte’s on the plain side, but Mary just isn’t attractive at all. (If she was, she might not hide in her role of scholar so much, don’t you think?)
May and Mr. Collins always struck me as the obvious match, but clearly Charlotte was scheming for him from the moment Lizzie turned him down. She wanted her own comfortable establishment and was clever enough to move in and flatter the dim bulb out of his hurt pride. Mary was pedantic, and rather admiring of Mr Collins, but she wasn’t bright enough to go after the prize, being the only daughter neither shameless enough to run helter skelter after men nor tasteful enough to not seek attention (although of a different sort). A pity, as in the long run it probably would have been a much happier (although much stupider) marriage than the Charlotte-Collins marriage probably ended up being. Charlotte knew how to manipulate him, while Mary would have not have.
Also, of course, Lizzie would have been less likely to visit Mary at Rosings than to visit her longtime friend Charlotte, hence the meeting with Darcy would have been less likely to occur. So plotwise, it was a better move.
Mrs Bennet originally offered *all *her younger daughters for consideration. Mr Collins had chosen Jane before arriving and meeting her hadn’t changed his mind - so Mrs Bennet had to hint that it was only the other four who were available. He swapped his preference to Lizzie It was soon done - done while Mrs Bennet was stirring the fire. Austen states that he’s fine with the change because Lizzie is equally next to Jane in birth and beauty.
Mary isn’t mentioned between Lizzie’s refusal and Charlotte’s acceptance. She isn’t even listed as present when all the girls are told to be quiet while Mrs Bennet chats to Mr Collins and Charlotte the same day as the refusal. Since Charlotte is happy to take his attention off Lizzie, whom he is treating with ‘resentful silence’, she is encouraged to spend more time with him.
The next morning he’s still in a state of ‘angry pride’ and the girls go into town to get away from him. It’s either that day or the next that they’re engaged to dine at Lucas Lodge and Charlotte is thanked for her efforts in keeping Mr Collins’ interest. By that evening, she’s almost sure she has him snagged.
It’s only a matter of a few more days before he’s sneaking out of Longbourne to propose to Charlotte.
Also, IMO, Mary wanted to keep her virtue. It was not uncommon in those days for one daughter to stay unmarried and be a helpmate for her parents.
I can only be glad they did not marry–their progeny would have been some of the silliest and stupidest gentry in England. 
I am not sure Mary and Mr. Collins would have been a very happy marriage. Mary does eems a bit of a snob. As Austen describes it…
Can you imagine her and lady Catherine together?
I have to say, poor Mary. Always felt sorry for her. Mr Bennett was more attentive to his elder daughters, and didn’t really pay attention to the others, while Mrs Bennett concentrated her focus on the two youngest and spoiled them rotten. They gave Mary an education, but that was the done thing; I think that, if this was a real family, she would be craving for attention. Consider the times she wants to play music in social gatherings.
Even Mary’s intention of saving her virtue and taking care of her parents could be seen that way; when all of her other sisters got married, she’d have her parents’ whole, undivided attention.
And with that, all three would be miserable: Mary because she holds her mother in contempt and thinks her father too light minded; Mr Bennett because he considers Mary too serious and lacking in humor and Mrs B because Mary does not share her liking of frivolous fashions and gossip. A pretty trio indeed…
I agree. I always HOPE Mr. Collins will marry Mary (I’ve only read the book 20 times, and it WILL happen one of these times) since they’d make such a wonderful unsufferable couple, but he never does. Because Mary isn’t attractive - Charlotte is merely plain - and because he is current mad at the Bennet’s when Charlotte makes her well timed play for him…
It works better both plot wise (gets Elizabeth to Rosings) and thematically. Pride and Prejudice is about felicity in marriage - marrying Mr. Collins off to someone like minded would not get the point Austen wanted to get across - that Charlotte was doomed to put up with an silly man for the rest of her life because that was the ‘surest preservative from want’ and she considered herself fortunate to get stuck with Mr. Collins. That point can’t be made with Mary.
When Mr. Bennet dies, Mrs. Bennet and her daughters will have 250 pounds between them. About a tenth of their income. If Mrs. Bennet can be prevented from spending the principal (yeah, right) when she dies the girl’s will each live on 50 pounds a year if they don’t marry. They don’t know how to cook. Lydia needs Sally to mend her muslin work gown for her. Mary is perhaps the only one of them that could make a passable governess. Elizabeth is not at all sensible to be turning down Mr. Collins.
I don’t know how unhappy Charlotte ended up being. I think for her, happiness in marriage wasn’t an expectation or a consideration. She probably got what she expected, and would end up pouring her heart and soul into her children and grandchildren. And she would find satisfaction and happiness in that, at least as much as she would in being a dutiful spinster daughter and sister, bowing to the whim of her male ralations.
It sort of makes you wonder what the Lucas’ marriage was like - Charlotte had no expectation of love, but at least three (including Lydia, although that was infatuation) of the Bennet girls marry for love, even though it appears there’s little love in the relationship of Mr. and Mrs. Bennet. It may be that, although they seemed to get on each other’s nerves, the Bennets were in love.
StG
I think Mr. Bennet may have originally been infatuated with his wife, when she was young and pretty and focused all her attention on him, but I think once he realized what a thoroughly silly woman she was, that infatuation died. Their marriage seems closer to Lydia and Wickham’s (except of course that Mr. Bennet isn’t a dog) than Jane’s and Bingley’s or Lizzie and Darcey’s.
Even given the financial security that Collins would give her, could she possibly be happy in such a marriage? She’d be much happier in genteel poverty.
Which is why I think it’s interesting that Mr. Bennett calls Wickham his favorite son-in-law. He’s being sarcastic, of course, but I think he also knows just what Wickham will feel toward Lydia in 20 years’ time.
Indeed, I think that what Mr B says to her near the end of the book (or the Colin Firth version) --something along the lines of “be careful how you choose; I wanted more for you than a loveless match.” indicates his awareness of the aridity in his own marriage. I think he’d rather see Lizzy genteely poor than unhappy in marriage. Realistically, the girls would have ended up being tolerated in Jane or Lydia’s household (if no Mr Darcy came along)–their lot would not have been happy.
Oh, I think Wickham will be feeling it sooner than that. He isn’t even infatuated with Lydia; she was just a convenient bit of fun for him and he got cornered. He won’t treat Lydia nearly as well as Mr. Bennet treats his wife, either, because Mr. B. is a decent (if neglectful) man and Wickham isn’t.