Is the length of the spell the reason why? Why can't a 4 level MU, every night after studying his spells, read the higher level spell and break it down so he could make himself learn it? What's to stop him? I don't see a real valid reason, other then its higher level and he just can't. I've heard its just the complexity of the spell. Well with a 17 or 18 intelligence, one would think that character after sometime, would figure out that complexity and learn that spell.
Anyhow, just some food for thought.