hello,
One of the condition for boarding is the refuelling.
Some places/countries like the uk are happy to board while refuelling. Certain procedures have got to be followed and a drill is in place in case of pb. I don't know if all airlines allow boarding and refuelling at the same time but mine does.
The country/airport may not authorize it so even if the cabin is ready pax may have to wait until the refuelling is done. This happens on longer routes when the aircraft can't take the fuel for the return journey.
Some countries/airporst may authorize the boarding while refuelling providing the fire brigade has a fire engine positioned near the aircraft. There may be a fee for such service so a lot of airlines don't take that option.