why do you think Draco and James are treated so differently for their bullying
I don't really think they are treated too differently. Both have stans who deny everything, but Draco admittedly has less people who want his guts.
I think this is for a few reasons.
One is the narrative's treatment of both characters. Draco is never treated like he's anything but a bully. James, by contrast, is set up as a good person and the narrative continues to treat him as such, despite very little evidence he even grew up. Yes, he has a few call out scenes, but they never address just how bad he was and someone is always very quick to defend him.
Another is background. Both are spoiled, had their magic nurtured, and very loved. But Draco is more understandable, as he inherited his prejudice from his parents and Lucius Malfoy doesn't treat people well either. You know exactly why Draco is the way he is. James? From what we see, his parents didn't raise him to hate anyone. He has a poor(and secretly werewolf) friend. His crush was on a muggleborn. These never cause problems for him at home. Maybe he was raised to hate Slytherins? But even so, we don't have enough information to safely infer that in the same sense we do for Draco's own behavior.
A third is just what they do. I'm not going to say Draco is harmless, because he's not. He very much bullies Neville and can be dangerous. But James? He sexually assaulted someone because Sirius was bored. He also made Snape at the very least jumpy, who unlike Neville, doesn't scare easy. Draco also challenges Harry to fair fights. One on one. James doesn't give his victims that curtesy.
Draco also suffered for his actions(not directly, but he was still hurt). Draco lived in constant fear for a good while. Voldemort lived in his house and were he to fail in reparations of the Vanishing Cabinet, his family is dead. Going back to the narrative thing, we saw him suffer.
James, presumably, got a quick death off screen/page. His death doesn't make us feel that much sympathy for him. It makes us feel sympathy for Harry. Yes, living in hiding couldn't be easy. But that isn't shown the same way Draco's fear of Voldemort was.