I am a gamer. I enjoy tabletop role-playing games, but I don't play or run the most popular of them all, Dungeons and Dragons. Why is that?
First, the initial expense is high. D&D's approach of making multiple core rulebooks, sold separately, at a relatively high price point, is excellent for their business, but I'm afraid I start to think at $30. I also dislike the implication that "the players don't need to know the rules" given by the existence of a separate book for those running the game. Players should know the rules, in my opinion; what they shouldn't know is the GM's intent.
Second, in D&D 4e especially, there's an ongoing expense. To get full benefit from your books, you'll want to get a D&D Insider subscription. Now, while this is an insignificant expense at $5 per month (assuming you get the most cost-efficient option, which is to pay for a year at a time; buy it month-by-month, and it's $8 a month), it's still a recurring expense, and while it's tiny (forgo tea at the knitting group meetings a couple of times a month), my mental block against such things starts at $0. This is why I don't play MMORPGs, among other issues (such as my addictive personality); I have difficulty dealing with recurring expenses for what I see as non-essential. Water, trash, phone, internet; I can justify these to myself. Even Netflix. But for D&D? Thanks, but no thanks.
Third, it's the 900lb gorilla of RPGs. While it's obviously done SOMEthing to merit its leading position, I resent its near-complete dominance of the market, and my previously mentioned dislike of the popular kicks in as well. I realise my contrariness is silly, but it's part of me.
Fourth, I just don't like it. This is actually the most important part; I've looked at the rules, and I don't like them, nor would I be able to easily house-rule out the parts I dislike. I'd rather start from a system that doesn't have the bits I dislike.