Why would Elon Musk pivot from Mars to the Moon all of a sudden?

As more than 120 million people tuned in to the Super Bowl for kickoff on Sunday evening, SpaceX founder Elon Musk turned instead to his social network. There, he tapped out an extended message in which he revealed that SpaceX is pivoting from the settlement of Mars to building a "self-growing" city on the Moon.

"For those unaware, SpaceX has already shifted focus to building a self-growing city on the Moon, as we can potentially achieve that in less than 10 years, whereas Mars would take 20+ years," Musk wrote, in part.