I think the main reason is that the people who use Mastodon tend to use it as a community to talk to like minded people, whereas the ones on the likes of Twitte

I think the main reason is that the people who use Mastodon tend to use it as a community to talk to like minded people, whereas the ones on the likes of Twitter see it as a free self promotion tool. And the people who follow you there also tend to post a lot too. Hence more engagement. Whether this lasts if influencers and corporations get into the Fediverse... well that's to be seen, but I suspect it'll probably resemble Twitter a bit more in that case.