Andy must have had a shower epiphany.
From his Reddit AMA 2 months ago.
I read something over a year ago from you that said you were doing a lot of research for your next book, which was supposed to be another hard science book. Later you said your pitch about a Moon Base was shot down, so you moved on to Zhek. I know Zhek will probably be a series, so there's not a lot of hope, but do you think you'd get to write the Moon Base hard scifi someday? And would you be willing to speak a little about what your idea was? I am desperate for more hard science fiction and I think it's a shame about the Moon Base idea, I thought it sounded really interesting (and you did all that research!).
Andy Weir's response:
The publisher loved the Moon Base idea's setting. They just didn't like the story. Some day I'll have a shower epiphany and have an idea for a better story and then I'll be ready to write it.