I believe this to be a very complicated set of factors... I'll try to mention a few, but not all of them, since I'm not old enough to know them :D.
- First of all, Texas Hold'em is the variation of poker with the easiest mechanics to understand. All others are a bit more complicated: in Omaha there are too many cards, in Stud games there are those weird rules about who acts first on each street and the draw games are just waay too weird and have the most complicated strategies (the draw games were actually the most popular at the beginning of the 20th century).
- The poker boom had definitely a big effect on this. MoneyMaker won the Main Event by playing Texas Holdem, so this is the preferred poker variation.
- Learning basic strategy of Texas Hold'em is probably much much easier compared to other poker variations
- Broadcasting Texas Hold'em on TV is a lot easier than with other variations. Think about a TV Show where people played Razz. All those cards would look pretty ugly crumbled there on the screen. Besides, the commentators have to remind people every 5 minutes that Razz is a Lo game and that the wheel is the nuts etc.
Texas Hold'em is definitely the most popular; so popular in fact, that most people don't even know about other types of poker. If you want to be a more serious poker player, you should definitely try other variations. Start with Omaha, since it's the one closest to Texas Hold'em. Learn a few strategies there. Then move on to Stud. Learn why that is such a different animal. Now here's the key: when you come back to Texas Hold'em, you'll realize that you actually became a much better Texas Hold'em player by learning the other ones.
PS: If Doyle Brunson sees this, please make an account and give us more historical reasons why Texas Hold'em is so popular. The rest of us are too young to know :) .