Duplicate
http://ae2.ae-mod.info/Features/0517/ - this is even your request, please do not post duplicates.
Two reasons,
1. Data density / Complexity; types are expensive to store and increase size of NBT dramatically especially in drives which can hold 10 of them. Having large NBT causes the game to take longe to slow, and and can bog things down.
2. Intended design, Storage cells consume 50% of the capacity of the drive for 63 types, if you exceed this it kinda means you can only store like 1 or 2 of each item... this is silly.