The government has done a pretty lousy job in rat control. They contract out rat control jobs to firms with dubious ability and reputation, hiring underpaid and non-skilled workers who really aren't motivated to do a good job.
To be fair, HK is far from the only major city in the world to suffer from rat infestation. New York, Chicago, London and Paris are all know to harbor a pretty substantial rat population. But even so, HK is where cases of rat-to-human transmission of Hepatitis E have the greatest frequency. That suggests the problem is more severe in HK.