Because she’s also the daughter courtesan I’ll be honest I have no idea how to spell that. And she has made it very clear that she doesn’t want to.
If I were to speculate since I haven’t read the light novel, it would prob be bc Jinshi doesn’t wanna be the Crown Prince/take up that mantle since he’s worked to distance himself from it by pretending to be an eunuch. If Jinshi were to ever get exposed, and Maomao’s position as Lakan (high ranking General)’s daughter, then it would prob signify and strength his position as crown Prince then eventual emperor, and i don’t think jinshi wants that path for his life.
If Maomao is the daughter of Lakan, a high ranked general, then why would it be an issue for them to be together? Lakan could just claim Maomao as his and it would be acceptable no?