Except we know from the Sorting Hat song that Slytherin valued Ambition and cunning above anything. Blood purity had nothing to do with it. Otherwise Percy would have been in that house.
Can't talk about Percy Weasley without talking about Arthur Weasley.
Arthur Weasley was a pure blood from a great house, but he was
the wrong pure blood from
the wrong house, according to the Slytherins. The Weasleys are a Gryffindor family as much as the Malfoys are a Slytherin family. The Hat barely needs to touch their heads to know these people. The Weasleys lived in poverty (even though Arthur had a high-ranking Ministry position) because Arthur (and maybe some of his ancestors) valued family and friends more than they valued money. Also, Arthur's Ministry job involves upsetting Dark Wizards.
Percy Weasley is tired of the poverty and the lack of respect (from certain crowds), and thinks his father has made mistakes. He wants to get into the Ministry and earn loads of money and be widely respected. He wants to be the "right" version of his father.
The Sorting Hat, I would say, sorts people based on how likely they are to succeed in a certain environment. If Percy went to Slytherin, he could make contacts and try to brownnose his way into a Ministry position, but the Slytherins would give him a rough time of it for the crime of being born a Weasley, and he might not make it very far. Whereas on the other hand, as a Gryffindor Weasley, Percy already has Ministry contacts through his father, which would likely burn if he showed himself to be anything other than his father's son (Sirius Black was kicked out of the Black house and had to crash on James Potter's couch after crossing the aisle to Gryffindor). Percy has enough cleverness
not to become a Slytherin.
He studied hard (a Hufflepuff/Ravenclaw trait) and followed the rules (disregard for the rules being described by Dumbledore as a sometimes-positive Slytherin trait) and was awarded with school accolades like Prefect and Head Boy (dismissed by his brothers as being too straight-laced). And then he graduated into a Ministry position.
And then, Arthur Weasley chose to do the right thing yet again and committed some more career suicide. And Percy said "I'm not with them" and chose to stand apart from his family. That was everything he thought he wanted, but it killed him to do it, and he immediately regretted it. He eventually worked up the courage to come back in tears and admit that his father was never wrong, that family
is more important than your career. And that, I would say, is why Percy's a Gryffindor.
