This happens because, in parallel computing, it isn't easily definable what it means for a variable to "persist". A peristent variable is normally one that retains its value from the previous function call. But when the objective function calls are all executing simultaneously (or at least non-sequentially), what does "previous" even mean?
With global variables, similar ambiguities arise. What if two parallel-occuring executions of the objective function assign different values to a global variable G that they share with the base workspace? Which of the conflicting values should reside in the base workspace version of G after the execution is complete?