JVM threads are relatively heavyweight. The JVM would fall over trying to run as many threads per machine as BEAM will happily run. And you still have the stop the world GC in akka.

Obviously that's not the be-all-end-all measure of a VM, but in the context of actors it's pretty damn important. Akka is essentially an actor library bolted onto a VM that really doesn't like what it tries to do. Some people make it work, but there's a lot of friction from the GC that gives rise to some really undesirable performance characteristics.