Because for a typical 3 bedroom house, you want about half the space to be bedrooms and half living/dining/kitchen area. If you put a bedroom downstairs, that

Because for a typical 3 bedroom house, you want about half the space to be bedrooms and half living/dining/kitchen area. If you put a bedroom downstairs, that means you have to put the living room upstairs. This is a big hassle if you have people over. Also by putting the bedrooms upstairs, you have a private space upstairs where guests don't usually go, so it doesn't have to be perfectly tidy when they come over.