From 1-50 for new players and existing players they have to speed up the process since they have to remember this game was out over a year now, by having slower leveling system that means they would need to concentrate on making those lower level contents more and that will give less for current players already at or near 50 on all classes. Even if you played only 2 hours a day if you started from day 1 of release of this game you would probably have multiple lv50 classes.

Pretty sure SE would love to make 1-50 slower but with how this game is already is now they want to put as much in to entertain the existing players, but it could also mean they might make 50+ take longer.

That's just my guess