20% of people are born powerless now. The implication is that all Quirks are not equal. E.G. not everyone has a Prime candidate hero Quirks like shooting explosions like Kachan. Thus it's very reasonable to extrapolate the actual viable "hero population" is likely much much smaller.
To put it in a real-world way, most people are able to play sports. Very few people are capable/talented enough to play in the NBA or NFL.