IMO the biggest reason is:

Starting out on any social media platform, _without_ the algorithmmic feed that all big platforms have, is hard.

Curating your own mix of accounts that are interesting but don't combine into a no-engage-worthy feeling fire hose of posts .. is hard work.

Many people prefer opening their app and scrolling through the latest hot shit, without having to worry how to manually get a feed made up of things that are relevant to them.