With any new platforms the base opex required to be compliant is a massive barrier to entry. You have to spend billions annually on content moderation staff, related tooling, datacenters, federal privacy and data audits, and all of this other compliance stuff to ensure that their platform isn't a cesspool and adheres to regulations of each country they operate in. If they don't, they'll never get approved to the Apple or Google Play stores, have the FTC all over them (plus EU is even more hardline here), have trouble getting reliable hosting if they don't build their own datacenters, and correspondingly struggle to add users or advertisers. It's a serious uphill battle and anything that would operate at the scale needed to support millions of users moving over en masse would need all that in place before the first advertising dollar rolled in, in a business where even most of the popular companies already hemorrhage cash.