Indeed, it seems quite extraordinary that someone could not be sent from one territory of the PRC to another one to face trial.
Beijing just unilaterally broke the Hong Kong handover agreement. In that agreement, China explicitly agreed that there would be two systems within PRC territory [1]. There is no way to sue the CPC in a Chinese court to enforce Hong Kong’s rights under the treaty.
In Hong Kong (as in Taiwan, Japan, Britain and the U.S.) the government is answerable to the law. In China, it is not. That is the crux of the issue. The way Beijing is cramming down this legislation is, funnily enough, the argument against it.
Someone has the great insight to foresee what could happen 30 years ago.