There are quite a few reasons, but they all boil down to the fact that Beijing wants Hong Kong to be part of China.
Culture: From what I understand, China sees itself not as a nation-state, but as a civilization-state. The government has a desire to unite all Chinese people together in a single nation. This is why they still claim Taiwan as part of China. Letting Hong Kong be independent is a step back from that goal.
History: During British colonial rule, China opposed all attempts at introducing full democracy to Hong Kong. It doesn't have strong democratic institutions in the way that you might expect. Yes it has a legislature, but half of the seats are essentially reserved for businesses (called 'functional constituencies'). The head of state is chosen from a pre-selected group of people who are loyal to Beijing. So even if the people all agreed on independence being the best path (and that isn't something that people agree on, as best I can tell)
Geography: Hong Kong is a relatively small geographic area with a very large population. It doesn't have the means to support itself. For example, most (all?) of its fresh water comes from mainland China. A lot of food comes from the mainland too. If Beijing doesn't want Hong Kong to be independent, they could simply stop supporting it with it's infrastructure.
Economic: Hong Kong has a thriving economy, and it's history has left it with a special economic status in several countries (including the USA). This makes it very useful to Beijing. They can skirt trade laws and funnel money through Hong Kong as needed. An independent Hong Kong could make this more difficult.
Culture: From what I understand, China sees itself not as a nation-state, but as a civilization-state. The government has a desire to unite all Chinese people together in a single nation. This is why they still claim Taiwan as part of China. Letting Hong Kong be independent is a step back from that goal.
History: During British colonial rule, China opposed all attempts at introducing full democracy to Hong Kong. It doesn't have strong democratic institutions in the way that you might expect. Yes it has a legislature, but half of the seats are essentially reserved for businesses (called 'functional constituencies'). The head of state is chosen from a pre-selected group of people who are loyal to Beijing. So even if the people all agreed on independence being the best path (and that isn't something that people agree on, as best I can tell)
Geography: Hong Kong is a relatively small geographic area with a very large population. It doesn't have the means to support itself. For example, most (all?) of its fresh water comes from mainland China. A lot of food comes from the mainland too. If Beijing doesn't want Hong Kong to be independent, they could simply stop supporting it with it's infrastructure.
Economic: Hong Kong has a thriving economy, and it's history has left it with a special economic status in several countries (including the USA). This makes it very useful to Beijing. They can skirt trade laws and funnel money through Hong Kong as needed. An independent Hong Kong could make this more difficult.