Evolution requires selection. Or in other words, having humans evolve into surviving in microgravity would require letting the least adapted people die of it for tens of generations. This doesn't seem like something that potential space colonists would be exited about.
Or we could select astronauts for longer missions based on these results. A person without legs (a double amputee, perhaps) might be able to perform all necessary activities while in orbit, but not suffer some of the ill effects.
