yeah when it works it's cool, but it doesn't always work.
Plus the user experience for anyone who not only isn't tech literate but neither fairly tech savvy can be a pain. Navigating the way into joining rooms, DMing people, etc is pretty confusing (even for someone like me, who can manually install Arch for example^[obligatory I use arch btw]) and that's without mentioning all the E2EE problems with federated instances, etc, which not only can mean sometimes users can't see each others' messages but it can even break bridges.
Oh and the bridges may be cool but they're also kinda shit, I mean the TG one can't handle supergroups, WhatsApp often requires you to open the app anyway to view certain content, I don't use the signal bridge cause I don't mind using signal. Oh and tbh bridges only exist because we still need to talk to some folks in the other places, but they do not provide you any extra privacy, your message is still going in telegram/meta's servers and through their apps (which realistically you'll also still need to keep installed anyway) etc and on top of that it will also go through the servers of whoever is hosting your instance.
Yes self-hosting is an option, no your average joe will not do it and it is absurd to ask them to, and self-hosting for your whole community is nice but it gets hard if you're on your own and good luck if you have quite a few friends to actually talk to that have to be in your instance.
Unfortunately, messaging apps are meant to do one job: talk with people, and in order to do that privately and securely you need to get people on the chosen app^[refer to part about bridges] and therefore it needs to be easy enough to grasp that even people who don't know what an OS is can get them, hence why Signal beats out Matrix (and getting people to switch to Signal from WhatsApp or Telegram is still pretty challenging).
So this is why I believe it's less developed. Matrix is probably not the only made in Europe project tbf, and besides even US things can be used if they are open source.
They're not mutually exclusive, do both