I think thread's number of users was because of how much instagram has made it's name in the social media market. A lot of the users were there because it was new so the larger userbase was already weak. On the fediverse however, you (at least right now) have to put in a little bit of effort to learn about federation, clients, services etc. It's there as an alternative to the bigger players rather than a polished packaged shitbox by them.

As @ObviouslyNotBanana@lemmy.world said, they will be there because of the invested time. I personally like it because it gives me close, tightly knit little communities on the web.