I don’t think that there’s anything stopping someone from making their own iNat focused Mastodon server (if I understand correctly, which I may not), just like there’s an unofficial iNat Discord. Running a Mastodon server does take a non-trivial amount of work and some time/resources, so it might not be something a smaller organization wants to take on in perpetuity or prioritize over other projects (I’m not speaking for iNat staff here, just my own thoughts).