Centralization is being driven by a lot of things, not least of which is that decentralization is really really hard. But a lot of it comes from mobile -- the idea behind decentralized apps before mobile came along was that bandwidth was not very dear -- oh, sure, it was limited in the days when IRC first became popular, but there was no reason to leave any of it unused. Local compute power was the same -- you may not have had a lot of it, but there was no reason to leave what you had sitting idle. Mobile is different. Idle CPU power can mean the CPU is throttled down to save on power and heat dissipation. Instead of staying connected to a server at all times, use "push" notifications so the server can let you know when it's worth using battery power and metered data to talk to the server.

Decentralized systems, and peer-to-peer systems, and even centralized systems designed around computers designed to sit next to a wall outlet and draw power from a practically unlimited source are all being replaced by systems that are more centralized and better equipped to talk to mobile. AOL Instant Messenger is dying because it was built around an older paradigm and, at least last time I bothered to try it, is an incredible battery drain on Android devices. Newer messenger apps that are built mobile first or at least make mobile an equal partner to PCs are winning instead. And it's the same for group chat -- IRC is built for old-fashioned PCs, Slack is built with mobile in mind.