Uneducated guess--somewhere someone will have archived the discussion--but I wonder if the technical complexity was also a factor. If you've made the combining characters for gender, and you've designated which characters can combine with them, but you didn't plan for characters that must be combined only in certain ways, then you're looking at having to specify all that out, and then implementers will have do their thing, and it just... wasn't worth it.

Uneducated guess, though.

EDIT: Googled. Here is a person who knows what was up: https://jenniferdaniel.substack.com/p/did-someone-say-new-em...

The submission guidelines have changed over time. I would assume that, at the time, the proposals _did_ meet the guidelines.

I don't know who is opposed other than those that want to pretend that pregnant transmen don't exist.