Because really, they are both rather plain. Shapeless and plain. I mean, I realized as a preteen that, based on the “beautiful” women on The Saint and Secret Agent, I should not expect much from British TV, but really, this is too much.
I can see where Billie Piper could be seen as plain, in a British sort of way, but I find her unbelievably sexy. I’m not quite sure what it is about her, so I suppose this isn’t a good answer for you.
I haven’t watched the Catherine Tate episodes yet.
I don’t know whether they were both supposed to be attractive in terms of the series, but I fiond Billie Piper attractive. Even more so in her Belle du Jour role.
Actually, I understand that. There have been women that made me feel like that.
Billie Piper is hot. Catherine Tate, not so much.
How do you feel about Freema Agyeman? She gets top marks too, especially in that impractical-yet-quite-fetching black leather UNIT get-up.
I fancy myself a man of good taste, so of course she’s a knockout.
I forgot to mention, Catherine Tate not so attractive, but Freema, very attractive.
Freema > Billie > Catherine, but I’d still bang them all, given the opportunity.
I haven’t gotten to either of the others, but yes, Rose is lovely. Maybe not an absolute beauty queen, but… golden hair, fresh-faced girl next door features, what’s not to be attracted to?
Obligatory reference to ‘Girl in the fireplace’ and the moment where Billie is tied up on the table by the repair droids, shapely bosom heaving and all. (Even in the commentary David and Sophia were laughing and pointing out what’s basically transparent fanservice) 
Martha is smokin’ hot: I’d place Sally Sparrow at the top of my doabilty list though - fwoar! I liked Billie Piper before she lost weight: she had that kind of dishevelled slutty hotness that made you feel that a couple of drinks and you’d be in with a shot.
Oh, Billie…
Catherine doesn’t do much for me though. Billie on the other hand…
You sir make a good point. Can I give you money now? 
This might be true of the Shrieking Harpy Bride, but Billie Piper? You need your eyes checked. She’s sex on toast.
Freema was a bit of alright, too, but Billie was just…fit. Yeah, definitely* fit*.
Oh, absolutely! I thought she, well, Carey Mulligan, had a shot at being a Companion. Little did I know that they were planning to tie the poor Doctor to another Jackie Tyler. 
It’s a shame that people seem to think that physical attributes is the be-all and end-all of attractiveness. If that’s so important, you’re going to be very disappointed in life.
The companions are attractive for the same reason why any woman is attractive: their personalities. Donna was an exception – she was supposed to start out as abrasive and shallow,* but she grew to get much more depth as she had her horizons opened. That’s the real tragedy of how she left the show.
She was also supposed to be a friend, not a potential lover.
*The type of person who would judge another on attractiveness, actually.
Hey! I resemble that remark!
I’m not judging them as Companions. I am happy to call Rose the greatest of them all, even judged against Sarah Jane and Romana, who were MY Companions. I’m giving Donna a chance–I’m only two eps in and am enjoying watching her grow. And the Doctor needs a Mate more than a mate. I just don’t find her physically attractive. That and I found Tate’s “Am I bovvered?” far more annoying than funny, so I was predisposed against her.
I always had a soft spot for Katy Manning’s Jo Grant: miniskirted early 70’s uber-totty. I can’t search for it now, being at work, but if you do a Google image search for “Katy Manning Naked Dalek” {Warning: NSFW!} you’ll get an idea of why…
Billie Piper is pretty hot in a sort of pouty Brittish kind of way. She’s supposed to be a regular middle class teenager (even though she looks almost 30), not an extra from Gossip Girl or The O.C. And she looks a little chubby in the show sometimes.
I was going to say Catherine Tate is not really that great looking. In the show she basically looks like a frumpy not completely unattractive, slightly bottomheavy middle aged woman. Then I found this photo and had to reassess.
They’re TV people. It’s ok to judge them on attractiveness.