The number of iconic superheroes introduced into the market began to dip in the 70’s because the way Superhero books were written changed. Prior to that characters were written as independent entities then gradually introduced into the large shared universe they were apart of. Characters had time to develop their own supporting cast and rogue’s gallery. In short they had time to develop their own individual identity before company crossover X took over their book for three months and their individual development took a back seat. It’s why new characters like Miss Marvel really don’t have their own rogue’s gallery.