"Why not? Isn't it up to God to decide who is and isn't elected?"
This is why:
"I am the way, the truth, and the life. No man comes to the father but through me."
This is elementary orthodox Christianity.
You don't know that Jesus is the way?
Calvinism teaches exactly the opposite. Accepting Jesus as your personal savior is completely meaningless, according to Calvinism.
You don't know that Jesus is the way?
But he isn't, if we are all pre-destined.