Because back in the day: raids = large group content.If people (in general) are only prepared to run 25 man if they get bribed to do so even if they prefer 25 man to the alternative, why have 25 man raiding in the game?
IF you ask me: the introduction of 10 mans was a big mistake. It was sorta o.k. in TBC but even then the transition from Kara -> Gruul/Mag was an dead end barrier for many guilds.
ZA being 10man also created unnecessary drama, peeps bitchin about being left out etc.
If Kara & ZA had been 25man raids, and 10mans were never introduced, we all would have a lot less problems.