Try to come up with someone that will generate buzz, that's PWI's main factor when doing this list, that's why kayfabe is used prominently in it.
In 2015 AJ was a 4 month transitional champ, used in the middle of the useless 2.0 version of the "Okada tries to be the new Ace trying to surpass Tanahashi" angle. And given how stables are filler more than anything else in New Japan, leading the BC isn't that big of a deal kayfabe wise.
Having said that, I have no clue who else could've won that year. But my point is AJ's case wasn't a slam dunk, kayfabe wise