I imagine it’s all to do with the perception of football fans and the concern over large events. The Portuguese authorities may be happy to open up the country to normal holidaymakers, they’ll feel happier relying on the cooperation of families, couples should they decide to impose any local restrictions etc.
Anyone who’s done a European away will know that the police and local governments don’t particularly like football fans being around. They don’t trust football fans and so don’t trust football fans to obey any guidelines they may set out. For them they may find it more palatable to keep fans contained and herded