is it because of the band episode or what?
The reason is not because in the first episode in which Porygon was featured in(Porygon the electric soldier) was banned. The reason was the reason it was banned. They have not had Porygon nor it's evolutions because they do not want anyone to remember the episode in which there was flashing lights that sent 800 children to hospitals. Other Pokemon have had banned premier episodes, but none of them had caused mass chaos.