Essentially, the reason HLA is still here bobbing along just fine and Plateia looks like a fucking graveyard, is HLA is mostly networked with the GNU social side of things and some matsoc instances that mostly use glitchsoc or older mastodon versions, while plateia was mostly networked with mastodon instances that keep fairly up to date, and Mastodon beyond a certain version breaks federation in a LOT of different ways, to the point where Plateia can't really reliably talk with most of them.  Just look at its network timeline :/