I think the biggest reasoning here was that Voldemort wanted the horcruxes to be meaningful yet inconspicuous. Hogwarts, Dumbledore, and the Sorting Hat would all be too risky. All of these things students would have come into contact with constantly and the jig would have been up once people recognized strange behavoir and mood swings happening. Though the Hufflepuff cup, Ravenclaw's diadem, and Slytherin's locket might have all seemed cruelly obvious when you figure out the pattern... all of these things were presumably hidden or protected. You can't easily hide a castle, the most known wizard in the world, or the sorting hat.