" As a passenger flight (BA027) operated by British Airways (BA) arriving from London of the United Kingdom to Hong Kong on December 8 had four passengers confirmed to have COVID-19 through specimens collected at the DH's Temporary Specimen Collection Centre (TSCC) while one passenger failed to comply with requirements specified under the Prevention and Control of Disease (Regulation of Cross-boundary Conveyances and Travellers) Regulation (Cap. 599H), the DH thus invoked the regulation to prohibit landing of passenger flights from London operated by BA in Hong Kong from December 12 to 25, 2020."
Source : HK government news website
here’s the govt reasoning