I like Samurai 8, personally. It's nothing groundbreaking and it's certainly not my favorite thing in the magazine, but I've read considerably more popular manga I didn't like as much, and relatively obscure manga I liked more, so it is up to an individual's viewpoint.
But yeah, people may not admit it now, but a lot of the hate when it first debuted was directed at "THIS is Kishimoto's big follow-up to Naruto?". Just because it wasn't this massively amazing thing they didn't love out of the gate, it instantly became flop material in their eyes.
Not all creators come back immediately with some amazing, successful series every time. They can't all be Rumiko Takahashi. Sometimes you have to write what you want a time or two and let it flop if it has to. This is what people don't understand. Samurai 8 is what Kishimoto WANTS to write, has always wanted to write. I'm sorry it doesn't align with your refined manga palettes that are likely no better than someone's who has only read a few series, but that's just the way it is sometimes.
Tl;dr a lot of it is bandwagoning or people legitimately not liking the series. Which are both fine. There's just no need to trash the series and be disrespectful of people who DO like it. Enjoy what you enjoy and let others enjoy what they enjoy.
Edit: also some people entirely ignore the fact that the ToC is in no way a 1:1 comparison between success or popularity, and that it's entirely possible they willingly chose to put it as a permanent back of the magazine (although I'm sure some people who clearly think they know better despite not being involved with Shueisha's editorial department in the slightest will cry and argue otherwise, or flat out not even bother to acknowledge the possibility)