Imagine if Bertie's family was all Slytherian and the Sorting Hat wants to put him in Griffindore. Even a young, first year Bertie would know how that would piss off his family, as the rivalry between the houses was always there, so he has to beg the Hat to put him in Ravenclaw or Hufflepuff, and the Hat knows that Bertie would give up everything for friends, so it sticks him in Hufflepuff. So Bertie goes away thinking he must be too stupid to go into Ravenclaw and doesn't bother studying.