How come Zilean isn't played more?

Honestly the utility in this champ's kit is astounding. Sure his passive definitely needs to be replaced, but honestly he's got a lot of power especially when paired up with immobile champs. I've started maxing his E first instead of Q and it really does the job. Slows down the enemy enough to land several hits and even set up for a stun. Or you can boost someone like Twisted Fate to land a gold card on an enemy. And then there's a lot of complaints about his bombs now. I assumed that you weren't supposed to really use them for damage and just focus on double stacking it on anything in a team fight. Some people not playing carefully even run into a missed bomb with a bomb already planted on them and stun their whole team on accident. And then there's his godly ult. With enough AP and CDR you can revive people upwards of 2k hp on around a 40 second cooldown if I'm not mistaken. I love the reworked Zilean and he seems to thrive with immobile champs that really do damage when they get to you while also having one of the most powerful stuns in the game on a low cooldown.