Draco was cunning enough by standards of eleven year olds. It doesn't make sense to expect them all to be mini Tom Riddles, or whatever people imagine Slytherins to be like.

 

Well, we know that Slytherin was a blood purist, and we know that the Hat decides people's houses on the basis of where the Founders would have put them, so it seems likely that belief in blood purity would land you in Slytherin.

People too often forget that the Hat is not a computer program. It does not determine house by automatic algorithm. It's not an objective analysis of a person's traits. The Sorting Hat simulates the four founders, including all their foibles and personal opinions. You can imagine that there would have been talented students all four founders will have fought over, regardless of their personality traits, and vice versa. And it would be entirely natural for the founders to make decisions based on random personal factors as well as personality traits. So you have things like families being normally put into the same house, you have children having their preferences respected, and you have Slytherin wanting the blood purists regardless of personality.

 

We dont know nearly enough about Draco to know what he was like as a person. Perhaps his ambition was to become the right hand man of the minister of magic, but avoiding the limelight (ala his father), and perhaps he was far more cunning than we ever see. The only time we do see his ability to be cunning is his plan in HBP, which Dumbledore didn't foresee. As for resourcefulness, for one I dont see why we discount his father, it was a resource and he used it fairly well, and in 6th year when he didn't have his father, he was still fairly resourceful (polyjuice potion, foresight to have lookouts, Rosmerta, ext). But we only see a very biased portrayal of him in the books, against someone he shares animosity towards, and barely seeing him for most of them.

Also, how would he fit any of the other houses better? He's not particularly brave, nor loyal, he doesnt strike me as being hardworking (although, again, biased perspective), and he doesn't seem like the type to enjoy learning for the sake of learning (which I internalise as being what Ravenclaw is, others might disagree). From what we see of him, he seems fairly Slytherin.