because it has to set up a virtual memory map before it can do
anything.   If you change that, you pretty well have to start all over
allocating where you are going to put things in virtual RAM and how
you are going to parcel it out for various purposes.

I had a similar problem in Abundance  which internally is not that
different from a JVM.  I did allow dynamic reconfiguration, and it was
quite a production to move everything and adjust every pointer in the
universe, and rehash everything.  There is a lot more stuff in there
than just the Java objects.  the system has to continue working while
you are busy adjusting it.  It is  a bit like repairing an engine
while driving down the freeway.