It dragged on too long.
First the edo tensin nonsense, naruto vs zetsu's, naruto vs jin, kages vs madara, obito vs naruto&Bee, itachi vs kabuto, then obito and madara vs naruto, then obito and madara vs SA, Then naruto and sasuke vs obito and ect ect ect
Too many Asspulls.
Minato sage mode, Sakura Byakugou, Naruto and sauke sage power ups, The sage himself coming back, The whole ootoosuki clan nonsense.
Not to mention the Shinobi alliance were more of a liability then any real help in the second half of the war. The only thing that stopped the war arc from being complete trash was Madara but even that wasn't enough from making the whole thing terrible really.