I'm not sure that the problem is missing manpower on the implementation side - it's more that it's hard to know as a developer which current blend of XEPs is the recommended combination and which might have the most chance of working between a given client & server (and server & client) combo.

Things like the XMPP compliance suite XEPs have helped a bit with this, but looking at the XEP list and trying to work out which XEPs you should be using on a given day is still daunting - as well as trying to track which clients are most likely to be supporting them.

The idea on Matrix is that you say "Hi, I talk Matrix CS API 0.4" and be done with it - and you end up with much more social pressure to keep up to date with the current latest spec, because otherwise you are simply falling behind (rather than happening to chose not to implement some XEPs).

It boils down to a question of governance & social dynamics rather than anything related to writing code (magically or otherwise).