According to Calvinists, why does God not predestine all to Heaven?

It seems to me that if God chooses not only those who will go to heaven, but those who will go to hell, a Calvinist would hold the position of Universalism, that being that all people will be saved. Why shouldn't they? If God is merciful and loving and decides the fate of every human, why would he send them to hell? Why doesn't he send them all to heaven since he makes that final decision?