I think for most of the reasons people have already cited are why a majority of folks don't buy 3rd party stuff. I think the biggest is quality. Some people probably got really, really burned by some really whacked out 3rd party product with bad editing, worse art, and atrocious rules. Certainly, there's plenty of stuff that fits that category out there. So, it's easier (and cheaper) to just prohibit all of it, if not a little reactionary, obviously.
The second big reason is research. Most people simply don't have the time to go out and read reviews, or keep track of what is available in the market, or what products might give them that thing they need or want. And since much of it is not available at the FLGS, they are limited to what they can find there.
My FLGS does seem to have a decent selection of 3rd party stuff though, so it's definitely out there and available.
For my part, I tend to prefer 3rd party stuff. My core game is the core books + the 4 core splat books, UA, and maybe some from MMII. The rest comes from the ToH, the Wilderlands Books, and (usually) Necromancer Games' modules. I use magic item creation rules (which I wrote) from Mystic Eye Games, and have also used Tournaments, Fairs & Taverns, and a host of other pdf and 3rd party stuff added here and there.
I find WoTC's books to be of a consistent "feel" and quality. The quality of a WoTC book, IMHO, tends to be at about average or above average in terms of content and rules. They're hard to beat in terms of art and production. 3rd party books can't rival them in that department. But I find 3rd party books often have much more freedom in terms of what they can do, and to me, the very best 3rd party books are way better than anything I could ever find in a WoTC book.