Date: 2017-12-04 09:53
I'd say the number 1 reason with beginners is poor teaching. Poor teaching that doesn't effectively address hand position, air, embouchure, and voicing, in approximately that order in relation to this particular problem. So many supposed difficulties are avoided with proper fundamentals.
Seabreeze, wouldn't you say that the left hand first finger functions as a register key when ascending into the altissimo? I would certainly agree that it's a more finicky register. Any weaknesses in fundamentals are fully exposed in that range, which presents a great opportunity to improve them!