Three Kingdoms likely did very strongly in Chinese market. Where as Warhammer is much more western brand. And well if some are happy enough with 1 or 2, and don't need to expand from there that also cuts sales.

I also recall Warhammer 3 having somewhat mixed reviews at launch owing to bugs and/or missing features relative to the previous entry. As you say, if you are already happy with what was available in Warhammer 2, why buy the sequel which is going to have its own DLC treadmill?