Same reason he was so famous anywhere alse. It started off because he was talented and cute as a youngster, then he was talented if a little weird as he grew older, but the weirdness sort of gave him more publicity and hype. The music industry thrives on the hype these days where a publicist, the right marketing, expensive video clips and all singing (mimeing) dancing extravagent live shows are far more important than your vocal talent and the message you are delivering with the song. Later on when his kiddy fiddling became an issue a lot of Westerners would have been turned away. I guess that stuff is maybe not talked about in China where it is thought to be better to just not mention some things. China is also a bit behind in some things because they had limited access to western music for a long time and because he is talented he now has a whole new set of fans. Goverment probably has no problem with his music as well because there is no political message to be found.

https://www.yahoo.com/entertainment/michael-jackson-apos-chemically-castrated-004406172.html

Castrated chemically by his father to keep his high pitch voice as a child.

That is sick twisted shit.