You#re right. There are many instruments that I would love to see in GarageBand iOS. But it is difficult to provide them all by default.
The update to GarageBand 2.1 added the ability to use audio units and instruments provided by third parties, so we can add the ones we want to use and omit others.
Now only the third party developers will have to offer additional instruments.
I think it is good solution to add instruments individually. A good virtual instrument needs a lot of storage for the samples and the storage on iOS devices is usually limited.
On my Mac the instrument library with all the jam packs needs roughly 35GB, not counting the additional Loop Library. Such a rich collection of sounds and audio material would not work on a small handheld device.