something I just don't get about Calvinism

some people say that Calvinism kills missions. I know that's not true. What I don't get is why.

I understand that if God chose people to be saved, then they will be. I get that missionaries will go whether they want to or not. I even understand that wanting to is an irresistible gift from God.

What I  don't get is why this isn't deterministic. It's no good calling it a mystery. Calvinism claims to explain everything.