Nepotism aside as the very clear answer, I just do not get how Carrie could be right for the part at all.
1) Calamity Jane is meant to be in the peak of physical fitness (lol, to put it bluntly, Carrie wouldn't fool anyone...)
2) The show is heavy on the choreo. I don't see a whirlwind like Jane standing still in Docs while everyone dances around her, but Carrie can't dance.
3) CJ is meant to sing with a country twang. Carrie can only sing straight in a shouty tone.
4) CJ is a plucky, strong but essentially chipper kind of gal. Carrie's smug smirk vs The Face gives none of this.
5) Everyone knows the show has a huge queer following. Why not cast an actor from the LGBTQ+ community?
6) Carrie has a baby. Her track record for missing shows isn't great anyway, but I see her doing less than half of the shows. Her understudy is in with a fantastic chance (look at where Georgina is now!)