At least partly because scumbags who want to abuse a dog until it's violent and dangerous don't buy shih tzus to start with. If you ban a breed because it's "dangerous" you'll see people defy it, evade it through hybridizing (which already happened, the bully xl is a hybrid of the already-banned bull terrier) or just start abusing and selectively breeding a different large breed until it's as violent as this breed. I'm not even necessarily against sunsetting this breed, it feels like we're in a situation where a lot of bad decisions we've made in the past have made this a good decision. But I think that there are a lot of core problems that won't be solved just by playing whack a mole with the current breed making headlines

Same reason Dobermans and Pit Bulls are the most abused Dogs in the US.