The first two of those led to purchases, and I will likely buy a copy of Iron Heroes at some point too. Even though practically speaking I'm not likely to play Warhammer or Traveller (though you never know), I still think I can learn from them. Despite the fact that I am interested in other games, D&D is the most common game, and this makes the language of D&D the shared language of tabletop RPG players. It is a common baseline that you can assume, much as you used to be able to assume that an educated person would have read Plato. (That's also why it's dangerous to create giant infodump settings, because doing so raises barriers to entry that are not present if you just say "Moldvay Basic" or "Third Edition Core," and it's why the development through play OSR dogma is so valuable.)
I've been running a Fourth Edition hack game for the past 8 months or so (2-4 times per month), and I'm just now getting to the point where I feel like I understand the system and its implications. And, despite all its changes, this is a system that still shares some DNA with the traditional D&D I am more familiar with. A game with fewer similarities (like, say, GURPS) would presumably take even more time. Not to mention the fact that you need to find other players that are similarly interested if you want to actually play. G+ and ConstantCon games make this easier, but still not easy (compare uptake of FLAILSNAILS and non-FLAILSNAILS games). In any case, playing via videoconference is still a poor substitute for playing at a real table.
This same dynamic is part of the reason why many people who are used to traditional D&D are turned off by Third and Fourth edition: they make the language less shared. They move or change the landmarks. They fracture the community not just in the sense that some like save or die and some don't, but in the sense that they change meanings. It's almost as if half your friends start speaking Japanese. You now have to learn Japanese too if you want to participate, and whether or not you think Japanese is an elegant or beautiful language, it is still work to learn.
There are only so many hours in the day. In addition to gaming, I'm heavily into weight training (a hobby that rules your routines like few others). I also like to read literature and history and spend time with my loved ones. I collaborate with some academics on social science research. Oh, and there is that pesky job thing that occupies 40+ hours of my life every week. When I do get to game, I want the rules to fade into the background.
To quote Noisms from Monsters & Manuals on new rulesets:
Nowadays, the prospect of picking up a 200 (or more likely 400) page-long tome of new rules fills me with dread and boredom rather than excitement, and I find that my attention span is only a fraction of what it used to be.It's not that I hate other systems or think that D&D is perfect (though I do think the pre-AD&D versions are pretty elegant), it's that D&D is the common tongue. Like English, we are stuck with it for now.