The boy does not belong in Slytherin.
Scorpius Malfoy self-identifies as a bit of a geek, and stereotypes aside, the Hat can see why, for the boy is highly intelligent and has not only an aptitude for magic but a fierce, encyclopedic interest in it. Scorpius Malfoy is brave and loyal and kind and would throw himself into harm's way to defend the ones he loves. Scorpius Malfoy is insecure and lonely, and he believes that the way through it is to belong to something bigger than himself.
Maybe that's why, even though the boy doesn't have an ambitious or competitive synapse in his brain, he asks the Hat to place him in Slytherin anyway: because all the Malfoys for centuries have been Sorted into Slytherin, and he wants to feel like he's a part of that legacy.
But the boy does not belong in Slytherin.
If only it were that simple.
Scorpius's father, and his father before him, were the exceptions—true Slytherins to their core—but the Hat would say that most of the Malfoy clan should have been in Hufflepuff, what with their utter devotion to their families and their fixation with belonging to an in-group. Scorpius here wouldn't do too badly in Hufflepuff himself, the Hat thinks, although Scorpius's intellectual framework of thinking is making the Hat want to see how he fares in Ravenclaw.
Generation after generation, the Hat plunked down onto the head of a shivering blonde Malfoy boy who prayed to be placed into Slytherin so as to uphold the family's honor, and the Hat obliged, hoping less and less each time that this boy will remember how it feels to fear disappointing his father, will assure his own son that he will accept him anywhere he goes. Skip to the next Malfoy, and the Hat's hopes are dashed when the story repeats itself. And on, and on.
The Hat doesn't have to keep putting them in Slytherin any more than it had to put Harry Potter or Peter Pettigrew in Gryffindor. But the Hat also knows better than anyone that some people don't have the courage or the self-assuredness or the social skills to face showing everyone who they really are.
It's a funny thing, seeing everything that's happening in another person's head, because the Hat can see not just that the boy belongs in Ravenclaw but also that he isn't ready for Ravenclaw. It can see Scorpius's nerves and insecurity and desperation to carry on his family's legacy so that he has something, something, to show for. It can see the contradictions in Scorpius's thoughts, the traits at war with each other, the parts of him that Scorpius is safer for them never to see the light of day.
The Hat can't see the future. It can't know if its choices will do more good than harm. It can't see if all those Malfoys really did find belonging and kinship in Slytherin, even if their sons were worse off for it. But it can see the present, and at the present moment, the boy is not ready for Ravenclaw. Maybe he would be someday, but that day isn't today, and today is the day the Hat has to make a choice.
It may only be a hat, but it picks up on things—the inter-house tensions, the Death Eaters. Would either of the wars have gotten so bad if the Hat had only mixed up the alliances a little? Would the Headmistress listen if it asked to do its job later, when the children have gained strength, or even not at all?
The Hat supposes that the question is, should it be placing a higher value on societal peace or on the frightened little lives placed into its care? Has it been insightful or negligent in failing to place any Muggle-born wizards into Slytherin for generations?
But the whole castle is waiting, and it is not the time for the Hat to have an existential breakdown over its purpose. It's time to make a decision.
It can only hope that it chooses wisely.