Why did he disappear?

Because TSM stopped playing him? Because Genja hasn't run him AD lately?

I'm not being cynical I'm just not sure what happened to him =P I didn't notice any hard nerfs, maybe some hit under the radar or something but such a versatile pick suddenly fading out into obscurity seems odd to me. He functions as a top laner, a mid laner, and an ADC, and he's not exactly an unsafe pick in any of the roles, as well as adding legitimate teamfight power. Not only that but you can first pick him, and leave your opponents guessing.

Is he suddenly just too weak or something? Other laners just do the laning better?

Who killed Kennen?

Exhaust did.

He stayed the same while they buffed others so now he is weaker in comparison but he is by no means a bad pick.

Nerfs killed him. That range nerf was HUGE. To the point where people stopped playing him.

Chauster made a post calling Crumbzz a r****d, stating the nerfs didn't affect Kennen but actions speak louder than words.

Kennen went from top pick prior to the nerfs to 0 picks in the season 2 championships because of the nerfs.

A bast... killed him. ;p
...That was terrible. Sorry.

No real reason, he's just fallen out of favor. Still a great champ.

I suppose the fact that he requires a lot of micro-management and doesn't have as much damage as other APs, but meh.