I don't know why CaptainCraig
is getting a hard time for pointing out the obvious. Hollywood is sexist and ageist! A man in his 40s, 50s or 60s can play an action hero in a franchise but very few women get away with it. Yes, there are exceptions to the rule but by-and-large, it's only established actresses, such as Angelina Jolie or Milla Jojovich, or else actresses who are associated with a particular role (Sigourney in Alien, Hamilton in Terminator).
Harmon's career never really rose above direct-to-video (and I choose 'video' deliberately above DVD, because that was her era). She's more famous for her love life than anything else now. The odds of her being handed the lead in a franchise like this are too astronomical even for C-3PO to compute.
And don't compare it to RDJ getting the role of Iron Man. RDS was always regarded as a great actor, even when at his career nadir. Angie - not so much.