I once read a comment on the effective altruism subreddit that tried to explain why aging didn’t get much attention in EA despite being so important, and I thought it was quite enlightening.
Longevity research occupies an unstable position in the space of possible EA cause areas: it is very “hardcore” and “weird” on some dimensions, but not at all on others. The EAs in principle most receptive to the case for longevity research tend also to be those most willing to question the “common-sense” views that only humans, and present humans, matter morally. But, as you note, one needs to exclude animals and take a person-affecting view to derive the “obvious corollary that curing aging is our number one priority”. As a consequence, such potential supporters of longevity research end up deprioritizing this cause area relative to less human-centric or more long-termist alternatives.
For background, here’s the comment I wrote: