Perhaps when the Army ever gets away from viewing the Helicopter as a variation of the Deuce and a Half Truck.....it might do better aviation-wise.
Then of course there is the age old bickering between the Air Force and Army over what kind of aviation assets the Army is allowed to operate and what limitations are place upon those operations.
Heaven forbid the Army should try to take on "real" aviation operations that would undermine the Air Force's perceived supremacy in such matters.
The benefit the USMC gets from being part of Navy Aviation is big budgets, sophisticated aircraft, and a more specialized mission than the Army.
Then there is the Leadership thing.....at least the Marines have Adult Leaders.