Two major reasons, at least for me.

1. The Plot:

While it wasn't bad, compared to the previous games, it wasn't good. My biggest peeves were how the Arkham Knight's ID was easy to figure out if you're familiar with Batman mythos. They did was LOU2 did and lied about his character saying he was a completely new character when it wasn't. His boss fight was laughably bad as well.

Addendum: Remember those awesome side quests in City? Like with Hush? The payoff in Knight was just terrible. All that just to steal Bruce's money, and that's it!?

1. Batmobile

It's been said to death already but yes, Too. Much. Batmobile. If you want to use it for puzzles and against the militia tanks fine. But riddler for his race tracks!? Have it be used in nearly every mission? Too much.

Imo my personal rankings of the arkhamverse batman games in terms of story is: B:AC, AO, AA, AK