Santa Ana was a man of monumental ego who believed his own idea that he was invincible. I expect that he more or less thought, “Well, if these gringo bastards want a fight, I’ll by God give them one.” I doubt he thought it would take more than a few hours to reduce the Alamo to rubble and slaughter the defenders. In his view, or so I believe, he regarded the Alamo as a minor inconvenience, if that. Plus, the defenders of the Alamo fully expected to be re-enforced; by the time it was apparent that wasn’t going to happen, it was far to late to abandon the place and save their lives. After that, I believe they decided to sell their lives as dearly as they could, except for the one guy whose name I cannot believe I’ve forgot----Moses ???; the only guy with a lick of sense, if my view. My Texas history teachers would crucify me for my weak memory and heretical attitude, no doubt.
Wars are not just a series of contests between several thousand armed men trying to kill each other. Regiments have to eat. They need ammo. Replacement troops. Supplies. And they can’t reliably scrounge for this stuff as they go. Especially in the deserts of southern Texas. They will need supply lines back to their home and those supply lines are vulnurable to attack.
Well, San Antonio is neither a desert nor in Southern Texas (hey, don’t feel bad, I used to think it was in the south too until I took a good long look at a map, insert jokes about public education in Texas and Oklahoma here). San Antonio is in the Plains region, if only just. Also, if they were concerned about losing the guns to the Mexican army, it might have been wiser for Travis to just destroy them and move on (if you’re in a hurry, you take a metal spike and hammer it into the touch hole as hard as you can. Congratulations, you are now the owner of a really heavy iron tube that doesn’t do anything. If you have some more time, you load it with powder, pack the muzzle full of clay, stick a fuse in it, light it and practice your sprinting.)
Oh, and one of the reasons the Texian troops under Houston had such success at San Jacinto was that they caught the Mexicans napping (literally, they attacked during a Siesta). It’s easy to pick on Santa Anna’s army for being so lazy and lounging off like that in the middle of the day, but I would like to point a small detail out in their defense, to anyone who has never had the distinct Karma-balancing experience of finding themselves in the general area of present-day Houston during the summer: It was late June, in a very swampy damp area near the Gulf of Mexico. They were basically hanging out in Hell’s own armpit, with temperatures likely in the high 90’s with 100% humidity. That place is damned unpleasant to be in at night when the temperatures drop to the high 80’s. 
But yeah, letting his army’s guard drop that far does not speak highly of Santa Anna’s abilities as a military leader (nor does losing a stand-up war to, well, the Texians.)
It wasn’t mosquito season. (Yellow Fever was a bigger threat than Malaria.) From Wikipedia; I can’t vouch for every word in the article, but this part matches with what I remember from other reading.
Note: San Antonio was “the political center of Texas.” There was a town out west, where El Paso/Juarez are now located; but that area was (& is) damn far from most of Texas. The only other settlements were pretty pitiful.
Santa Anna got his supplies the old-fashioned way - he brought them with him. He had a mule train following his troops. He wasn’t transporting supplies across Texas from Mexico. So what were the Texans going to attack?
Thank you for all the insights. So, like most puzzling situations, it’s a variety of reasons: Big egos – on both sides. Poor planning – on both sides. Things not going as planned, such as they were – on both sides. Looks like it wouldn’t have taken much for the whole thing to have never happened and we’d be talking about the big clash between Santa Anna and Houston’s armies a couple weeks earlier than it happened.
Sorry** Nemo**, I forgot about this thread. You don’t want a mobile artillery unit to your rear. If you engage another force you will end up trapped between them. It would have made more sense for Santa Anna to leave enough of a force behind to make sure the men at the Alamo were not mobile than to attack them and take casualties at several times the rate he delivered them.
One of the things that always struck me about the Texas War for Independence is how few people were in the armies. The Texas army numbered no more than a few hundred. The Army of Operations was maybe 2000 poorly trained and motivated troops.
During the Mexican American War which was fought a couple of decades later, Mexico fielded around 40,000 troops and the U.S fielded twice that. During the Revolutionary War, the British fielded as many as 40,000 men in their New York operation that chased Washington across the Delaware river. Heck, the French had 10,000 troops in the U.S. Why were there so few troops in the two forces?
I think the big part of it was just that the Texas Revolution wasn’t a very big war. There weren’t a lot of settlers in Texas yet at the time, IIRC. Also, given that there were only a few hundred rebels, it wouldn’t make much sense to take a huge force of troops into Texas. It’d be like bringing a huge sledgehammer to kill grass hoppers.
Plus, given that there wasn’t much of a population base in Texas to support Santa Anna’s army, he had to bring his supplies with him. Your supply tail gets bigger faster than your march gets longer, as the supply tail also needs supplies to support itself, and men to protect it, depending on where it is moving through.
Probably the only reason the war ended when it did was because the Texians had the good fortune to capture Mexico’s head of state (who had been so obliging as to come all the way out into Texas where they could get at him.) If he hadn’t insisted on marching into Texas himself, the war might have lasted longer or ended differently.
In any case, the Mexican-American War was simply a much bigger war than the Texas Revolution was, since it was essentially a full-on invasion of Mexico by the United States in order to secure Texas and sieze the real estate between the US and the Pacific.