I pretty much restrict myself to WoTC supplements (and Paizo products) when it comes to rules. I would consider buying some 3rd Party modules etc (like DCC or Necromancer), but Dungeon covers the majority of my needs. There are four main reasons for my purchasing patterns:
1. Distribution/Marketing - WoTC products are higher profile and are stocked at more locations. Given the various higher profile marketing channels available to WoTC, I'm much more informed of their products than I would even of the upper tier 3rd party publishers.
2. Supply - Related to #1, WoTC products are pretty much available in any major bookstore, not to mention my FLGS (which have fluctuating supplies of 3rd party products). I'm lazy, so it has to be pretty special for me to go out of my way to get the product.
3. Support - Let's face it, WoTC products are better supported in the area where it counts for me, adventures. You got Dungeon pumping out high quality short adventures every month that incorporate material from all of the various sourcebooks, WoTC is moving into adventures in a big way (hopefully, most will follow the Dungeon/Red Hand of Doom design philosophy), and even 3rd party publishers support the core products. We're even beginning to see cross-pollination between WoTC's non-core sourcebooks. Look at the examples provided in The Player's Guide to Eberron, Sharn: City of Towers, and Monster Manual IV. Oh yeah, and there's Dragon which continues to support the new rules with various articles 12 times a year.
4. Design Philosophy - This is a personal thing for me. I buy WoTC products because there are some core philosophies that carry over from product to product (core d20 rules, campaign portability, options for players, making the game easier for DMs by providing pre-gen resources, etc.). I find it easier to fit in rules designed under the same philosophy than a 3rd party product that significantly diverges from the philosophy that I'm used to and have come to expect.