There are two versions to explain this:
The first one is more about rumours and thoughts, but based on previous ***events*** and comments, the MP devs want to push this version of multiplayer to the masses and want to use the early access time to polish it, before having other mods and custom servers do it's own thing and gain popularity. Back in the beta there was a group of people or a single guy, can't remember really, that had custom servers working until TW shut it down in an update, there was also a comment from a dev (I think, there's a screenshot somewhere) that said that their work wasn't being credited properly and that the custom servers were being called the "better ones". Do not take my word as 100% truth though, this is only what I remember it said, nor I know the context either, but well it seemed as if the devs were kinda wanting to limit the custom servers so they don't get their work to waste, which is something that it's very likely to happen, accounting the state of the MP (afaik, people don't play Captain or Skirmish anymore, only siege and DM at night with constant server crashes, at least in NA) .
The second version, given by Taleworlds is that the custom servers are on its way but that it was going to take some time and that it was very difficult (again I remember they've said this, it's in the forums somewhere, I would search it but man I'm in the bed and I don't wanna lol). It was kinda weird, because clearly Bannerlord Online is using their own servers with no problem, and a dude in the beta was pulling it off on its own, I don't know how servers work but it's kinda odd not even MP mod support is out there, maybe they're waiting for EA to end, maybe they are truly working on something that it's taking a lot of time, that statement was made like a year or more ago.
There's no way to be sure really, but it's been too long and the fact that the servers crash all the time in NA after 2 years in EA it's kinda ridiculous, It's very weird they haven't set their minds and finally fix that issue, I wished a dev answered what the problem really is
And yeah OP, in fact I was googling PW in Bannerlord today lol, I'm eager to play it too, it's very sad mods can't do anything because of TW cutting their legs off, those mods are very popular and I have no doubt they would explode, I would play all day