There are actually many reasons for the Chinese to restrict mainlanders from easily going to Hong Kong, but as I understand it one of the main ones is the lower sales taxes in Hong Kong would allow ample opportunities for arbitrage/resale of goods that could be bought in Hong Kong and then easily driven/walked back over the border.
Also, the Brits in Hong Kong don’t generally make a pain in the arse of themselves (unlike the EasyJet hordes around Europe or on all-inclusives in the “Dom Rep”) so there’s no practical reason to repeal that particular status, other than pointless spite of course.
