That's generally a factor of Ichigo's blandness. He's written like he's got some kind of crazy charisma that can turn enemies into allies, like Goku or Yusuke or Luffy, but we're never shown that at any point in the series.
It's also a factor of Kubo just introducing new characters when he gets stuck plotwise. Oh, Ichigo can't open that door? Let's introduce a character whose only purpose is to give him the key to the door. As a result the cast bloats to insane levels.
Even ballooning cast is just a symtom rather than the issue I feel.
The only recurring important part of the cast is Ichigo, everything centers around him and relates to him in a way.
All the other characters serves no purposes but to prop strawberry boy up, they could be interesting but they're usually given so little time to be useful and they're so inconsequential that they're just filler for the buildup anyway.
Ichigo is flat like a plank and is never shown being that exceptional anyway (in a funny way, the only reason he was important in part 2 was because he was NEVER SHOWN the special skill the villain had...which proved pointless anyway).
It's constant buildup that go nowhere.
It's a major problem for manga because it makes reading a chore once you know that.
Even with ballooning cast it could have worked, that's the whole point of the "villain of the week" format after all.
Heck even Pokemon manages it fairly well, the hero is a bit flat but it works because the main character isn't a void of personality that we are given no reason to care about.