OP, here is the reason:
Being a Sasuke fan, or an Uchiha fan on this base means a few things:
1. Hyping/Justifying/Advocating for Uchihas in all cases regardless of circumstances and regardless of who is right/wrong/weak/strong/or anything else. Uchiha fans don't care about the innocent. They just care about Uchihas. They don't care about truth or lies. They just care about Uchihas. They don't care about justice unless it's justice for the Uchiha. Etc... One prominent Uchiha fan around here once said that he would approve of the Hyuuga clan being slaughtered for no reason by Ao, but he (along with many other Uchiha fans) was always up in arms when it came to the Uchiha massacre.
2. Seeking/Hoping/Advocating for the failure/suffering/destruction/undermining of any clan or individual that is not Uchiha. Be it, the Hyuuga, the Otsutsuki, the Senju, the Uzumaki, Kakashi, Minato, etc... If it's not Uchiha then Uchiha fans will bash it. Uchihas absolutely must be superior, and anything that even remotely acts in the favor of another clan or individual must be undermined.
3 (The most important one) Uchiha fans must always want the worst for Naruto/Boruto and anyone related to them (Hinata/Minato/Kushina/Himawari, etc...). Uchiha fans always want them to fail at something, to be weak, or some other bad thing.
That is what it means to be an Uchiha fan. (This is not to say that this is 100% of Uchiha fans, but a large majority of the prominent ones that stand out in my mind are like this).