It’s a result of the much higher level of commitment required to undertake crewed Mars missions. You can’t lean on Earth being nearby and contingencies have to include e.g. uncrewed resupply missions failing… it’s the sort of thing you have to go all-in on, after which pulling out is costly and looks stupid.

By contrast, backing out of the moon is comparatively trivial. At worst it’d take a couple weeks and the expenses quickly lost in the noise.

With Mars, there’s also the possibility of a “Wild West” element once things are established enough where groups of people simply refuse to return and cutting off support from them would have really bad optics.