There's physical limitations on what it can do, basically.

Every mutation runs the risk of reducing its binding affinity to ACE receptors to get inside cells - or might impede cell entry functions.

Given the amount of infection out there, whatever "easy" mutations might've been available to it clearly haven't been very useful in increasing its rate of spread - and haven't touched anything the vaccines target.

So it's quite likely that whatever needs to happen to evade the vaccines is otherwise deleterious to its virulence.