For me, it was the interface. I found it rough around the edges and not as inviting as Twitter used to be. I know it's seen as superficial but UX/UI is important.
Like, for example, to create a post or reply, the input was on the left navigation panel for some reason. I used to have trouble visually separating one post from the next in my head until I got used to it. Also, the way thread comments were nested could've been improved. And why did it only show me the top 5 trending news stories? Why couldn't I browse more? Idk, overall I felt like I was fighting the UI mentally.
I think Lemmy did a better job subtly improving on the details. I didn't see Mastodon doing that much when I was on there.