I feel like I'm in this group, and although the jury is still out, my personal anecdata makes me think it's one of two things.

(1) The virus is possibly hiding in the body creating these little reservoirs that allow it to persist.

(2) Some people are likely just more susceptible to damage caused by the inflammation or whatever in the system and just take longer to recover.

I have a feeling that it's likely a combination of both of these.