Because it is the largest and most important D20 site on the internet.
The sense of community here is great - I can post to Gary Gygax and he'll answer me - and the same can be said for virtually any other member of the industry. I know - as I've done it often.
How cool is that? Quite simply, this is the place to be.
I enjoy my hobby and have for nearly thirty years. Not surprisingly, thousands of others feel the same way and have been enjoying it for about as long as I have.
Funny enough - they come here to post about the same things that I do.
RPG.net has a higher noise to signal ratio, is less friendly and more snooty when it comes to game discussion. Perhaps because of the swearing, it just generally feels a lot less friendly a place. The WotC boards are a disaster for the most part.
I enjoy industry gossip, campaign updates, rule discussions, product dicussions, story hours, polls on which employee of Paizo to eat first, whether Battlestar Galactica was good or bad this week and - of course - I like showing off the Coolest Gaming Set-up. Evar. and grin broadly every time I see a new post by someone else who has taken the plunge into tabletop projection, etc..
It is, quite simply, the best place to be on the internet if you like D&D.