The biggest issue is that the rmb is not even freely convertible under the capital account. Until the rmb becomes freely convertible, Hong Kong dollar and Hong Kong stock exchange still have their special functions.
The RMB is not freely convertible and that's by design.
During the 1998 financial crisis in Hong Kong could the HKMA have defeated Soros and his cronies without Beijing pledging not to devalue the RMB and ensuring liquidity for it to soak up a market in free fall?
If you want to debate HK's GDP, RMB Clearing in HK makes a tidy sum for its economy, back in the day, London fought tooth and nail to get the rights to do RMB clearing in Europe!
Switching to stock markets, having a listing somewhere allows companies to raise capital if you are a Chinese company needing to raise capital then domestically RMB convertibility is a non-issue, if they need to raise non-RMB capital, for say offshore expansion, that can be done in any stock exchange after the organisation meets that exchange's criteria.
Besides how much capital is raised as equity and how much as debt through bond issues by Chinese companies? Before we go down that rabbit hole China's bond market is bigger than HK's!
Do Chinese companies prefer Hong Kong, probably but its not because its 'special'. Looking in the opposite direction wrt investment how many of China's foreign acquisitions have been of HK listed companies?
Hong Kong's financial markets provides a function once upon a time it was crucial but it's no longer that special or unique wrt China, what it does do is provide Hong Kong with a means to pay its way. Its not to say its not important but now a days its merely one amongst many.