ActivityPub (AP) has the advantage of running on top of HTTP. You don't have to convince people to install a new browser. All the tools that communicate over AP run on familiar technologies. The hardest part is convincing people to make an account, so most growth is from people following friends--the key incentive--to Mastodon instances. It's not a big problem in practice. Monoculture concerns aside.
AP doesn't have to run on HTTP, but any alternative has the same problem as anything else that needs its own client. Even browser support for FTP is on the way out, and it used to be standard.
