There are millions of people playing MMO's these days and many of them have slightly different tastes. 10 years ago you could only spread those people acorss a handful of games, so the sub numbers were high. Now you have at least 10x as many MMO's, all fighting for roughly the same amount of people.
Of course sub numbers across all games will drop, it's simple math.
This doesn't even get into the other aspects that also affect MMO subs like the social and generational differences between the core gamer 10 years ago and todays core gamer.