Simply put, because modern family jokes are higher and colder than the Big Bang Theory.
The jokes in "The Big Bang Theory" are relatively straightforward, and most of them come from the strange words and deeds of several nerds. You don't need to know much about American culture to understand these jokes, as long as your neural circuit is different from Sheldon's.
The jokes of modern families are more subtle, and most of them come from the background setting of uncoordinated behaviors of several protagonists. To understand these jokes, you need to know something about American culture. For example, gay, played badly by the screenwriter, is a white terrier (Jay owns the closet company, Orz'). Conservative white men like Jay will say something that sounds normal but gay from time to time, and Phil will "pretend" to be gay once or twice in every episode. Audiences who know a little about American gay culture are easily amused when they see conservative straight men talking seriously about gay jargon, but those who don't know can't understand these jokes. Manny, as a boy, most of the plots that happened to him were like adolescent American girls. For example, Manny talked about a girlfriend named Sam, and Jay took the initiative to find Sam and told them not to move too fast. This is usually what an American father said to his daughter's boyfriend. When the audience saw Jay, the father, seriously warning his son's girlfriend not to go too far with his son, the joke naturally came. There are also many immigrant stalks used in the play, such as Cam's scenes, Lily's poisonous tongue, Alex's eternal bookworm attribute, and Haley's chest is big and brainless, which are all rooted in American culture.
Modern Family is destined to have a smaller audience than The Big Bang Theory, so it is normal that it is not as popular as the latter in China.