Maybe it was a dumb question for me to pose. If there isn't an accepted canonical answer, and no definitive studies, there is way to answer it. For example, all these answers sound vaguely plausible:
1. Principle land dancers perform for hours at a time, with relatively short breaks. Skaters compete for a few minutes in a day. Do adults have better endurance?
2. Because the expectation is that some land dancers will be older, companies may look for dancers based as much on future potential as on one or two short performances. I.e., it is their potential as adults that counts, not their current skills.
3. Because of that same expectation, elite level dance teachers may be more willing to devote their time to teaching adult dancers. Maybe elite level skating coaches are less eager to do so.
4. Maybe the posture constraints of figure skating, like gymnastics, are oriented towards youthful forms, at least for the girls. Whereas maybe it is easier for an adult women to meet the aesthetic constraints of ballet, modern and contemporary dance. I don't know this to be true, but it's conceivable.
5. As near as I can tell, there are probably tens of millions, at least, of little girls who take up ballet and other land dance with some degree of seriousness. As near as I can tell, that's at least 1 or 2 orders of magnitude more than take up figure skating. Plus many land dance studios and dance schools offer discounts, scholarships or informal work/study to their most promising students, so families don't have to be as wealthy. So, by the time you are looking at principle dancers of major dance companies, the girls are at least one or two orders of magnitude more select than the girls who grow to win Olympic or Worlds' gold figure skating medals. Those who audition principle dancers can choose girls with perfect (for the activity) post-maturity body forms (eliminating, for example, "curvy" upper bodies), and can choose girls with an absence of prior major injuries and conditions. Elite level skating coaches have fewer students to choose from, so can't find girls whose bodies are ideally suited in the long term - maybe.
I bet you folks could come up with more explanations.
I don't see how one can answer the question definitively unless there have been definitive studies. Unless some of you know of such definitive studies, it was a dumb question to ask. Sorry.
And adding in the gender variable was pointless. Too many differences.
P.S. Katherine Healy is an exception to EVERY rule. No one person can do all those things.