That's also where your argument from before comes in, as I don't use matrix. Never had an use for it, that's where the conflict for me comes in not wanting to create an account on a platform I don't personally use.
Mattermost I will have an use for personally and with friends, while matrix for me is unnedded and overkill for my needs, that's why I didn't go with matrix, neither on an already established server, or self host it.