Generally, it's because the student, for one reason or another, isn't enjoying the training or they're not getting what they're looking for (which may or may not be the same as what they
claim they to be looking for in MA).
1. Money - School costs? - if someone really thinks it's worthwhile and what they want to train, then they'll find a way around this (all the worthwhile instructors I've ever met will work with students on this problem)
2. Instructors/Students - relationship problems? - the student ends up not enjoying the training because of the relationship problems.
3. Better schedules? - this is a possibility but, usually, if it's what the student really wants, he/she can make priority changes and manage the schedule (not always possible, of course, but more often than not)
4. Interest in other arts? - if the student completely leaves the school because of this, then it probably means they weren't getting what theywanted in the school
5. My school only had ugly babes! - again, not getting what they want out of the training
Like I said, it generally boils down to the student not enjoying training for one reason or another.
This is why, when newbies ask what they should train in, I tell them, "It doesn't matter. Find an instructor and class you enjoy. Because if you don't enjoy it, you won't stick around anyway. After you get some experience and training under your belt, then you can look around to see if there's another system/art that you might be more interested in."
Mike