Because people are people. You won’t be able to get on the train to commute when people are sleeping there.

People don’t value free things, and it’s hard to plan things without pricing demand.

Because people in America have being taught to think that way. Plenty of countries run free (or near free) public transit just fine.

The price is not the barrier for most people using transit. The service reliability and availability is more important. Would you rather pay fare for frequent bus or wait around for free fares?

Because the tube will be filled with the unwashed masses

I answered more thoroughly in response to your sibling comment.

Because socialism.