In the hands of a really capable developer, JavaScript can be used to engineer reliable, complex systems. Sadly, most people who write software for a living are not "really capable", so the extra safety net provided by compile-time type checking and good IDE support that comes with Java tends to outweigh the concision and flexibility of a dynamic language like JavaScript.

Also, while V8 is quite heavily optimized for performance on modern PCs, I haven't seen much to support the idea that it is a great fit for memory and CPU-constrained mobile devices. Early versions of Dalvik specifically traded runtime compilation (as implemented in HotSpot and most other modern JIT-ing JVMs) for ahead-of-time in order to minimize the memory footprint of applications. Any VM designed around heavy runtime optimization (like a mobile V8 derivative) would have to be carefully tweaked to not thrash horribly given the memory constraints on most smartphones.