Pathfinder's not better, it's just different.
Not that different, mind you, which is why I don't see the point in switching over.
This.
And basic inertia:
1) I have a metric ton of books.
2) One of my players yelled at me for frequent edition changes/making him buy new books (we started with AD&D in 1998, then 3.0, then 3.5). I don't like taking money out of my player's pockets, since not everyone has the same resources in real life.
3) I know the rules. The players (mostly) know the rules. Learning new rules would take away from playing, and thinking about playing. Rules aren't the point of the game, at all, for me and my peeps.
4) Not broken. My players never did the bad wrongfun things that made 3.5e broken, so we don't see a need for a fix. We were never CharOp folks.
I do buy plenty of Pathfinder stuff though. I want to support Paizo, and I like reading the modules & adventure paths. Perhaps someday I will convert them, or switch to PF.