They can now. At the time this was not offered because the CCP was so against it.
In order to get the 1 country 2 systems deal the UK had to shelve the idea at the time. It was a massive gamble but it was probably worth trying for a peaceful hand over.
Since the CCP have broken their side of the agreement there's no longer a reason not to offer BNO citizens a route to UK citizenship now.
[1] https://www.gov.uk/british-national-overseas-bno-visa