I understand what you’re saying. The desire of St. Isaac to return is huge, and religious no doubt can make requests, but he wasn’t going to go anywhere unless he was sent. That sending is special too.
Today, I believe there are missionaries like St. Isaac who want to go to peoples who are resistant to the Faith and who would most likely kill the messenger. Today, a missionary might make his request to go to Iraq, or Libya, or Sumatra, or Jolo, but would most likely receive the response from his superior that any Joe Sixpack would make, i.e. “Are you crazy? Too much unrest. What value would you be if you’re dead?”
Don’t take my word for it. Listen to the Archbishop of Baghdad (Babylonia) when he says, “Where are the priests? Why does no one come?”
It is not for lack of desire, but permission to go.