Most reasons have already been listed. But the biggest of them all is what levis said above, with the combination of younger people not choosing nw but rather holdast or roblox campaigns.
Lets hope bannerlords multiplayer servers come out soon, so a numbers rebound can be achieved in S&M.
S&M is still moving ahead at a good pace but the server issue worries me a lot. Which is something I have asked multiple times yet get varying stupid responses.
I hope history will repeat it self, and im sure when servers are out, the community will make bannerlords multiplayer great. S&M included. Again that is what made Warband such a great game. The huge communities, the diversity of mods, tournaments etc. It basicly has something for everyone and thats why it is still alive today (ofc smaller than the past, for the many reasons discussed above).
Bannerlord multiplayer will be a breath of fresh air. It has a very big player base many mods already beeing developed, its the hole package for a success and for it to be the continuation of our lovely NW community. But damn realisticly right now Bannerlords multiplayer is horrible, the game has still quite some work ahead of it on singleplayer. And we have no clue when to expect servers or their size of them. I too am worried about the server issue.