The communists kept HK around to facilitate some trade with the west as it closed off from the world (otherwise they could have easily “liberated” it in the late 60s). With the opening of China by the 80s, a UK ruled HK didn’t make sense to the PRC anymore.

HK never had a legitimate claim to independence being a colony. Taiwan is a completely different case since it is ruled by locals (Chinese) rather than some colonial overlords.