For me, a lot of the 3rd party stuff I have is on topics that WotC haven't covered (or hadn't covered at the time I bought the 3rd party book). Often these are on fringe topics that I find particularly interesting.
More options is another reason why I have bought 3rd party topics. I bought the Complete Book of Eldritch Might to give me more spells in my game. The Spell Compendium wasn't out at the time but I have since bought that as well. You can never have too many options. Along a similar line, I bought the book the Tome of Horrors because I wanted more monsters to add to my game. I don't think that MM3 was out at the time and MM2 was v3.0.
Adventures is another area that I have bought 3rd party books. I own most of the Necromancer line, a lot of the Monkey God adventures as well as a few from Goodman Games, Fiery Dragon, Malhavoc and Green Ronin. Apart from RttToEE, the only WotC adventures that I own are The Sunless Citadel and The Forge of Fury and I only bought those 2 adventures in PDF a couple of months ago. Having said that, I own quite a lot of Dungeon magazines which you could consider as WotC adventures. For a long time WotC weren't making adventures so I had to go elsewhere to get them. Now that they are back in the adventure market there are several I want to get including the Red Hand of Doom and Expedition to Castle Ravenloft.
Price is another factor for me. In most cases, 3rd party products are cheaper than WotC products. This is especially true for PDF products, which account for a lot of the 3rd party products I own. WotC PDF's are sold at close to full price, which makes them unappealing to me. In most cases I could buy a print copy and get it shipped to Australia for less than the cost of the PDF.
Having said all of that, I have noticed that I have seemed to have drifted back to buying a bigger percentage of WotC books in the last 6 or so months. Also, apart from adventures, most of the books on my to-buy list are WotC books. I think this is as much to do with the fact that there are less 3rd-party products around in general these days, apart from adventures, which still seem to come out at a pretty good rate.
I still think WotC puts out good books. It is just that sometimes I want more options or they don't seem to want to put out a book on a subject that I'm interested in.
Olaf the Stout