It's the AD items that are just very good on him. Why build AP?
AD has much better itemization. It scales better, not just on him, but everyone else. You get life steal, better split pushing, movement speed, and dueling.
Last year, people were building AP kennen... it's the items that changed..
LocatedEagle2321 votesWell.... His abilities scale with Ability Power. That is the sole constraint.
If you feel you can get more use out of stronger auto attacks with your stun and empowered lightning attack, then AD can be very viable!
AP is still a great option from his good ratios and strong ultimate! (And his stun can be used to suppress enemies while your AD carry wipes the floor clear :D
xJLx MCHammer1 votesWell.... His abilities scale with Ability Power. That is the sole constraint.
If you feel you can get more use out of stronger auto attacks with your stun and empowered lightning attack, then AD can be very viable!
AP is still a great option from his good ratios and strong ultimate! (And his stun can be used to suppress enemies while your AD carry wipes the floor clear :D
When you have a champ that can scale from both AD or AP, people will generally building the one that is much better
Last year, Kennen was being build as AP, especially in worlds. They nerfed reliable tanks items and tank champions like Mao, Naut, and Poppy. This enabled duelers and split pushers. Obviously, Kennen will be taken and he will be build with what is better (AP or AD).
AD items are just too good now. AP kenen has decent ratios but the only thing good about it is that you can get ta Zhonya. With AD you can get -GA -Lifesteal -%Max Hp -Better slows -Get of cc card (QSS) -Harder to itemize -Better wave clear -Movement Speed
All this means is that Kennen has a clear choice
LocatedEagle2321 votes[{quoted}](name=xJLx MCHammer,realm=NA,application-id=3ErqAdtq,discussion-id=dEPtRcpX,comment-id=00000000,timestamp=2017-06-17T01:47:46.694+0000)
When you have a champ that can scale from both AD or AP, people will generally building the one that is much better
Last year, Kennen was being build as AP, especially in worlds. They nerfed reliable tanks items and tank champions like Mao, Naut, and Poppy. This enabled duelers and split pushers. Obviously, Kennen will be taken and he will be build with what is better (AP or AD).
AD items are just too good now. AP kenen has decent ratios but the only thing good about it is that you can get ta Zhonya. With AD you can get -GA -Lifesteal -%Max Hp -Better slows -Get of cc card (QSS) -Harder to itemize -Better wave clear -Movement Speed
All this means is that Kennen has a clear choice
Well, first off, I appreciate the downvotes....
If you think AD is better, then play him AD. I just would prefer to play an ACTUAL ADC who has much more qualification to be built with the items you like so much.... I basically main ADC and I have played AD Kennen as well as AD Thresh, and AD Bard. HONESTLY I can tell you that Bard ADC is better in my experience than AD Kennen.
(OK so first off, you start Spellthiefs, poke in lane, collect meeps when possible and have a support that is self-reliant, then rush Hextech Gunblade and some mobility boots and then grab.... that one gun that gives you the increased attack range. I then get an Infinity Edge or Statik Shiv (preferably both of them...)
doodlebob6661 votesthe ult change is one of the main things that pushed him to ad, it made his ult damage very unreliable, since this was so much of ap kennens damage that nerfed the playstyle very hard, adding to this the changes to 
, made ad kennen top stronger,
Tormentula1 votesNot necessarily just the items, kennen got nerfed a couple times last year after his dominance in competitive play. That and ADC kennen became a strong pick early this year in competitive, and after the strength it provided, it showed it was also a much better pick top lane.
Kennen's autos were always since the beginning of time strong enough to allow him to play a marksmen role. It was never not a thing, just unviable for the longest time until it became the go to meta build for kennen after AP nerfs. Kinda like ezreal mid, kennen ADC existed it was just a really weird or bad pick for awhile, but with adjustments they could easily which roles/builds as their kits fit perfectly in them.
Sir Fuzzi1 votesHis AP stats suffered, his AD did not, his AP builds have changed around and generally have less AP per capita (Rylai and Zhonya used to be more). AD items have been on a rise lately too so there's that issue.
It's all the factors but the items are certainly a big part of it too.