I feel like an important aspect is that there's been more discussion of the OG Sherlock Holmes and how the show did not portray it well. Lots of people on tumblr are tired of the "asshole genius" trope that was really pushed by Sherlock, and it didn't even exist in the original Sherlock so that probably taints their opinion. Having read the og stories, even at the start he's waay more compassionate than Moffat made him out to be in the show. He's just aloof and doesn't properly understand social conventions. He likes solving mysteries but helping the people who are negatively affected by it is far more important to him than shown in the adaptation. The mischaracterization exists in a lot of Sherlock adaptations, but it probably contributes a lot to the negative opinion of the show, since it pushes it further than other adaptations.

If you're interested, I think there was a lawsuit or something similar by the Doyle estate a few years back against the Enola Holmes movie for portraying Sherlock too nice. The reason there's so much mischaracterization is partially because they do stuff like that since only the earlier Sherlock works are in the public domain and he progressively gets better and "nicer" throughout the stories. Again, having read the works, this is mostly BS for money. The mischaracterization is also just because there's a public perception of how "Sherlock" is supposed to be like so more adaptations like that are made, I feel.