I hear you, kinda, but… Is it Mastodon’s job to promote other services? Mastodon doesn’t even implement ActivityPub to specification, so why would you expect them to hold hands and sing kumbaya?
I will say this though, a.gup.pe and chirp.social both should’ve probably switched and became Lemmy instances. As someone that follows !Firefox from my mastodon, I can tell you the user experience for the two kinds of groups wouldn’t get worse, but only better. Especially with 0.19.x and the changes made to Markdown support. Users would be able to access the groups via both Mastodon and Lemmy and not be limited to the Mastodon UI seems a no brainer to me, especially since it’s a drop in replacement.
Edit: I will add that dickheads are dickheads. That has nothing to do with how active somewhere is. Also it takes time for a community to grow. Things will go up and down, but the general direction remains that there’s more users here than a year ago.