I do not know, but if I had to guess, it is for performance.

I am no expert in this field, but AFAIK http://static.usenix.org/events/vee05/full_papers/p153-yunhe... (discussed on http://news.ycombinator.com/item?id=704576) has not really been refuted.

I base that on the fact that both Dalvik (http://davidehringer.com/software/android/The_Dalvik_Virtual...) and Lua 5.0 (http://www.lua.org/doc/jucs05.pdf) use register-based VMs.