I still can't, to this day, pinpoint the exact reason the manga went downhill for me, as I remember finishing it and feeling absolute disdain and thinking that all these years of loyally following the manga were a complete waste of time. But I will still try to list a couple of possible causes:

(Spoilers ahead)

1. Lousy, poorly executed fights. Bleach is a shonen manga, a BATTLE manga, but the fights were never its strongest suit. I understand, as neither is One Piece (my favorite shounen manga) that phenomenal with its fights, but having to achieve a new transformation/unlock a secret power EVERY SINGLE TIME Ichigo is struggling with an opponent got pretty dull. Fights could be interesting when the protagonist ends up winning from sheer will, determination, or tactics, or when their opponents are generally on equal footing with them, making the fights more logical. With Bleach it was never like that. Every time Ichigo was defeated we had to brace ourselves for a new transformation that more often than not felt like an asspull or it was poorly executed. There are just so many new forms Ichigo could unlock before we all got sick of it and the hype had died down. Ichigo had a baffling identity crisis by the end of the series because no one knew what he was anymore and it no longer mattered. We were so desensitized to him constantly unlocking a new transformation that made less and less sense as the series went on.

2. Horrible plot points and mustache-twirling villains. The motivations of the main villains were shallow at best, and laughable at worst. (I say main villains because Ulquiorra, Grimmjow, and Kenpachi as side villains, were done well). Aizen's 'I planned this all along' still irks me to this day when I think about it. I still can't fathom the existence of that plotline. It did nothing but totally disregarded every single development that Ichigo went through and made it all seem meaningless, like the suspense we felt throughout the journey was for nothing. Yhwach's true identity came completely out of left field, as every single interaction that Ichigo had with Zangetsu made little sense and again, disregarded everything that the two went through together. Building a better relationship with his Zanpakto. The rain. Zangetsu's fusion with Ichigo's hollow form. It all meant nothing because OH, IT WASN'T HIS TRUE ZANPAKUTO ALL ALONG. As for Byakuya, I still don't understand how smoothly he became a 'good guy' just because it was revealed his true intentions weren't to kill Rukia all along. He's still the one who hunted her down, brought her to Soul Society, and was overseeing her execution himself. Getting his full backstory served to only make his motivations more and more senseless. I like Byakuya, but the reveal was so poorly done I still wonder how the hell his relationship with Rukia improved so much. And Kugo? I don't remember a single thing about him. The whole arc felt like a blur of nonsensical events.

3. As a main character from a shonen manga, Ichigo lacks individuality. Thing is, "protecting the people I love" isn't a concrete goal. Let's compare him to the MCs from the other big three, now granted, Luffy and Naruto have their own selfish goals, but that is 100% ALONGSIDE their motivation to keep their friends safe. Protecting their friends, saving innocent people, and overall making the world a better place is a given to them. Luffy literally goes around different islands cleansing them of evil, tyranny, and injustice just because it's the right thing to do. Naruto is a ninja, whose sole purpose is to help the world become a better place and keep it in order. Their goal of becoming the Pirate King/Hokage is a desire they selfishly have because it gives them more depth as characters. Having a character who simply wants to 'protect their loved ones' or 'do the right thing' is shallow, as there must be a driving force that gives them individuality when there isn't any danger to said loved ones. The truth of the matter is, if presented with the choice to protect their loved ones or achieve their goals, both Luffy and Naruto will choose to protect their friends, which is literally the reason it takes them so long to reach said goals. Doing the right thing comes first to them. THAT'S what makes these characters relatable. Having their quirky personalities and unique goals is what gives them individuality. Ichigo lacks the latter.

4. Various characters, various factions/organizations, multiple dimensions, but all with no proper world-building. Kubo absolutely excels at designing characters. Out of the big three, Bleach had the most intricately drawn and awe-inspiring character designs with unique abilities, however, it doesn't help that he constantly liked to create a faction of characters, make them all beautiful and badass looking, hype them up, then completely neglect them when their story is over. Think of One Piece, with the Shichibukai, the Yonkou, the Supernovas, and how each of their introductions were grand events in the series and their role continued to be important even decades after their first appearances. Bleach doesn't have that, unfortunately, as the backstories of these side characters are often vague and the source of their powers is left unexplored, which is a shame. The reason I got into Bleach was the unique (and definitely not stupid looking) characters.

5. The anticlimactic ending. I don't even need to expand on this point. Yhwach got increasingly overpowered as the story reached its climax, to a point where everyone was sure there was absolutely no way to defeat him. Yhwach being defeated the way he did made everyone feel cheated, and we must admit that this was the main source of the community's dissatisfaction and hatred toward the series, which I can't, logically, claim it's undeserved.