Why don’t people like CinemaSins?
Short Answer: They're bad at movie criticism.
Long Answer: Their premise and branding has forced them to become bad at movie criticism in order to continue their livelihood.
CinemaSins began as a channel talking about bad movies, pointing out their flaws, and counting it up as a sort of ranking.
The problem is that it became popular, and because it became popular it became monetisable.
All of their problems really stem from the fact that they earn a livelihood from it.
The Youtube Algorithm started focusing on longer videos, so the videos had to become longer, which meant more sins. But even the worst movies have so many flaws, and you can only make the "joke" sins so many times. So the criticisms started getting... looser. Sinning things that get explained moments later. Sinning things that aren't explained, but are obvious without explanation. Sinning things that are completely subjective.
At the same time, there's only so many popular bad movies. Plenty of completely obscure ones, but nobody's gonna click on a video for a movie they've never heard of. So they started sinning good movies, which left them less to legitimately criticize, while still needing longer videos with more sins. Worsening the problem.
And then the selection becomes more limited when you start chasing what's popular. There's a sequel coming out, got to make a video on the previous film, soak up some of those searches. Doesn't matter if the last movie was a masterpiece of cinema, gotta sin it.
And because the channel was popular, it's word was parroted. It gave a platform for some worst aspects of film criticism: the focus on "plot holes"; the conflation of the two definitions of "critical"; the constant "why didn't the characters do x? lol dumb movie" spiels... all existed before CinemaSins, still exist even in those who profess their hate for the channel, but they became the loudest voice making these points.
They're not the first to fall into this trap. The wave of Caustic Critics spawned by the Angry Videogame Nerd suffered this as well. You cannot monetise negativity forever, there is only so many genuinely bad works of media that are interestingly bad, or praised as good and are actually bad, or bad in a way nobody's discussed before.
If you keep telling people "X is Garbage and Here's Why", you're gonna run out of garbage and start having to manufacture it, no matter what the actual facts are.
Other creators adapted. Found other ways to make their content interesting besides anger and negativity.
But CinemaSins can't. It's in the name.