Imagine you start a company that makes vehicles. It turns out people really like your cars and trucks, but bikes not so much. Sure some people bought them and even advise you how you could improve them but you already made huge profits of the first two products and it's simply not worth it to take care of that small but demanding portion of customers...
This is exactly the situation with Bannerlord. There is some old screenshot from discord of conversation with a dev that explains mp crowd is tiny enough to simply not matter (they even mentioned that shutting down mp altogether wouldn't mean much in terms of financial loses). Even now the game is Very Positive on steam and you all know it's because of SP.
Personally I think that is all there is to it, unfortunately. SP feature came out better so the resources are focused on it. MP came out worse, so it's left to die out. That's it.
The real "cruelty" here is something different though. It's actually the existence of this forum.
Think about it: there are pages to no end of all levels of insults towards the developers with close to no response either on the forums or in form of actual development of things mp people want. The things that actually get implemented with the little resources left for mp seem to be really disconnected from the rude feedback. In every other game forum insults of this caliber would be moderated into oblivion, but here they are not. Try say something racist or sexist though and the reaction will be swift.
Why is that? It's simple:
The very lax moderation in regards to insulting developers is to provide a discrete place for venting frustration of unfortunate players that ended up in the "unimportant" portion of customers. I think TW deliberately allows flaming on this forum that most customers doesn't even know about so that the game doesn't get bad comments on for example steam or reddit or whatever main mass of players uses. It's a place for all you MP passionate gamers to cry till you die and you do it out of sight of other people that actually enjoy the main features of the game.
Now you know what you all gonna do?
"LLALALALALALALALALLALAA didn't hear that, TW when will you implement XYZ omg u suck as devs"