Ageism especially against women. Famke also called this out a few years back on how ageism and sexism were why she wasn't being asked back for future X-Men films after DOFP.
https://www.theguardian.com/film/201...en-replacement
Cassandra's age makes even less sense if she's still supposed to be Xavier's twin like the leaks are claiming unless she comes from a very specific period in the timeline.