A few nerfs + season three changed things. People would rather try out new stuff to begin with, then they find things that seem like they work well and try to maximize that potential with other champs. --- Deo evicto eum occidam ut se quendam deum fiam.
He didn't benefit from S3 items as much as other top laners. --- Actually, since Wedge Antilles > Carriers, and Carriers = Instant win, then it should be Wedge Antilles > Instant win. He's just that good XD -PsionicDude
he's less popular due to nerfs(no longer OP) and he isn't popular in competitive play. I also think people have stopped playing him because it's not so easy to land his EQ. In addition, those EQ combos do eat up quite a bit of mana on an already mana intensive champ.
Honestly, I still enjoy playing him and feel that he's still a great champ when played correctly. Before nerfs, he was extremely strong at multiple areas so you really could have any playstyle and not be punished hard for it. I absolutely enjoy Jayce's low popularity because ppl don't know how to play against him anymore
Jayce simply doesn't do enough damage anymore AND has mana problems. Honestly, he's a bit underpowered at the moment. Honestly, you need to forsake some damage just for him to not have to go back to the base every 2 minutes --- According to Wikipedia, you don't exist! CLICK HERE FOR FAME AND GLORY