Good question really.
This is only vaguely related, but if you have ever played A Tale In The Desert you will know a bit about the levelling system there. Basically it takes so many resources and so much time for someone to advance in one of the seven disciplines that you often find people concentrating on one branch only at a time, and it is rare, even amongst people who have been there several months, to find one master at one branch let alone advanced in more than one. I could imagine the system here being not so far different - the lower, functional levels of the magic system being attainable with moderate effort but requiring massive effort to excel in one, and godlike to excel in several.
Why advocate such a difficult levelling system? Same reason I advocate expensive items - the difficulty it takes to obtain them are their very worth. If you could purchase a Galkard in one days worth of gem hunting then why bother? And if you could become a master of all the Ways in a week the game would get very boring, very quickly...