then its probably because most other champs are better for the team comp or has a better strength. Take riven....actually no don't, take......idk......shen. He's a super tanky ninja. So all he has to do is buy thornmail and armor and then wait for his teammates to come up and finish jayce and he'll be dead. Maybe even malphite. Malph usually only gets taken if there's also a yas mid, but combine those two and screw a jayce when we can just 1v4 or 5 the enemy team.
He is powerful, but he's more like sona where he does a bit of everything and no one big thing he does like malph (tank and ult), or shen(tank and ult xD). For those reasons, I think Jayce just doesn't fit as well as others do in their teamcomps or such.