Because Judal was never really on 'our' side while Sinbad was. To Sinbad's perspective, he's only white. After all, he's only doing what he's doing for the sake of his country. (but I haven't really read the latest chapters of Magi so I can't say much)
That's why i'm saying that people don't read with care. Look at this: http://www.mangago.zone/read-manga/magi_sinbad_no_bouken/mh/v03/c029/1.html/pg-14/
Sinbad doesn't say explicit "I'm gray", but if you read you can understand it. Sinbad's perspective is not he's only white. Like i said, i'm not finished Magi yet, so i need to do it for a global perspective, but for the things i read until now i can say that the hate from most people in the fandom is irrational.
I cannot explain really good in english so i'm apologize if this can be reading offensive. It's not my intention ;___;
Since you don't know everything that Sinbad has done in the main manga, I will tell you this. Sinbad has done things that firmly put him in the corner of "you need to stop and chill holy shit". This including betraying several main characters, making an alliance with one of the most hate character of the series and slowly but surely cutting close ties with his companions.
Judal on the other hand hasn't done anything nearly as bad as Sinbad by comparison and he's always been the huge jerk who is a mess from the get go.
I'm sorry for my bad english and for the explotion of emotions below, but i have to say it:
I'm reading this before end Magi cause i want understand why people hate Sinbad so much. I don't get people, Sinbad is not bad, he says it himself: he is not white or black, he is grey. Yet, people hate him like he was black and an enemy... WHY. But if its Judal..., then everybody loves him. Just for his past. REALLY? THAT'S BULLSHI! ;___;
The past of Sinbad is sad as well. I don't get people, really!