Money, BIG MONEY ... as in TRILLIONS of dollars. You can read about it here:
I doubt the Elon Musks of the world want to go to Mars because it will be a great adventure. They want to go there to mine the riches of the asteroid belt.
A warning to the wise - government funds "invested" in such ventures ultimately will only make rich people even richer. Like most "investments", space mining is not likely to benefit We The People in any big way.