Ok, for one, it is easier for Japan, Philippine, Taiwan, and US to do "Houthi in Red Sea" than China in that area. The sea is closer to their land, compared to China. So any China ships and aircrafts that come to that area will have to face a web of Anti Air and Anti ship missiles.
Secondly, it is easier for US to plant sea detector / sonar in that area than China. Of course China can do it in peaceful time. But in war time, it is easier for the US to plant military buoy to detect Subs and ships than China. It is also easier for them to plant anti subs underwater sonar than China. Plus, it is also easier for US and her allies to deploy subs in that area than China.