[{quoted}](name=Roit Gums,realm=NA,application-id=3ErqAdtq,discussion-id=d6GlQHY9,comment-id=0000,timestamp=2016-09-28T20:45:10.228+0000)
Because the armor pen build late game is really good vs tanks you can easily kill them, while crit might be good yea but vs tanks it will be normal like any other adc without the pen
Its worth noting, Crit does end up scaling significantly better into the lategame (due to his low AS since he has to reload his gun and fires very slowly; Jhin is pretty bad at killing tanks even when he stacks armor pen compared to other ADCs)
Lategame Jhin can hit around 800 to 900 AD with his crit build and Lord Dominik's Regards; which makes him able to kill pretty much anything in a single reload or less, except the beefiest of opponents (who will usually die after a second round of bullets). 100% crit also enables him to activate his passive every shot.
TL;DR: Armor Pen Jhin is better for fighting targets early on, Crit build scales better; most games end somewhat early so armor pen is usually better (though I've noticed games are starting to last a little bit longer on average recently, from my experience anyway)