Given the core model of objects that are independent computing engines (storage being instance vars, CPU being the class methods) responding to messages passed from one to the other, it would seem that Smalltalk would be a natural fit for parallel processing on massive numbers of cores. Yet this is an area where Smalltalk is still very weak indeed, replying on its own simulated multitasking features which do not exploit modern processors' hardware capabilities.
Why is this? What are the main problems? Is mutability the key, or is it something more specific to Smalltalk?