Because not everyone plays for gear. 25 mans are fun most of the time.
All they have to do is throw in mounts or special titles for 25 man and that'll motivate all the gear-chasers to go back to 25.
I'm so out of this world, I make telescopes squint.
-=702 Gamer=-
Because, when you raid a 25m, more of your guild can progress, lets say you have 2 10m raid groups, and only one of them progresses, then you would be leaving a large part of your guild behind. And i know a lot of people that would choose 25 over 10.