The most populated servers in Warband were indeed private servers with a wild variety of maps. Having official servers, and a form of progression (armor, xp, gold) tied to it, completely disencentivized players from populating private servers in Bannerlord. If there was a private siege server that was regularly populated in Bannerlord you would also see a lot more modded maps (of admittedly varying quality. It not having to be official also meant there was little quality control, but that was almost part of the charm).