Matrix shows how thorough the switch to HTTP has become. Like with DoH, what makes Matrix most attractive is the ability to implement solutions using pre-existing web application stacks, like Node.js. It's a sort of path dependency where the easiest way to implement a simple solution starts with a very complex, mostly superfluous foundation.

The fact that port 80 and 443 are less likely to be blocked is a distant, secondary factor. Frankly, I've never been on a network that blocked other outbound ports. And I've definitely never seen a network where hosted services could only ever receive traffic on port 80 or 443. Rather, I think it's more (if not entirely) a matter of people justifying the use of web stacks to implement services which cry out for simpler solutions.