For me it was because of there being more character development, proper foreshadowing, and strength levels making sense.

Character development being the big one. You CAN'T just throw away Aizen and expect Bleach to still be any good anymore. Aizen was the enemy that should've been the end of this series - he was properly built-up and everything. You can't just toss in a new set of baddies that nobody heard of before (Quincies) just to keep it going.

It's also why Naruto fans didn't like Kaguya - really came outta nowhere and Madara should've been the end of it.

And it would be like One Piece continuing on even after the Gorosei have been defeated. It would get criticism if that were to happen.

Or, using OP as an example again, the battle with Doflamingo was amazing. Wanna know why? BUILD-UP. He was introduced WAAAYYYY earlier, and been a real bad-guy and chaos bringer since the start, and further it showed his and Dressrosa's past before the final showdown, making Luffy vs Mingo not just about fists, but about ideals, world views, and even "The Will of D" vs "Tenryuubito".

But imagine if Doflamingo wasn't built-up. That you only just met him in the Dressrosa arc, there wasn't much explanation as to what was going on in Dressrosa or it's past, and the Strawhats just eventually picked a fight with them and defeated them. You wouldn't like it nearly as much, in fact you'd probably think it's shit storytelling.

That's what Bleach is right now. Just a series of battles with no real foreshadowing or buildup. It's just bad storytelling.

You can't just pull things out without proper build-up like that.

Furthermore, Bleach stopped having the good moments it used to. It got "too" serious and "fighting ALL THE TIME" and now there's no focus on the characters outside of a fight setting. No dialogue or funny moments or anything anymore.

All of this makes it easy to get detached from a story.