Currently HK has extradition treaties with 18 countries, but none with Macao, Taiwan, and mainland China.
On the face of it, it is rather odd that currently someone may be extradited from HK (a territory of the PR China) to, say the US, but not to mainland China.
It is understandable that people have reservations because of the judicial system in mainland China but this should ideally be dealt with by safeguards in the proposed bill.
To me it is a bit like arguing that someone in Puerto Rico should not face extradition to the US mainland.
That's why it's a question of integration. If HK were fully integrated with the PRC, requiring an extradition treaty at all would be strange. But it isn't integrated and the protesters evidently want to keep it that way.
I don't know how many Puerto Rican separatists there are, but they would probably oppose extradition to the United States.