Short answer: I would say mostly cause the math is easier :lol:
Also it can be a lot harder to get equipment for graphing, cutting etc odd angles and that most quilts are made for beds that are a rectangular shape. Although I have seen some interesting shaped quilts, Star shape for a chrismas tree skirt and I have made a hexagonal quilt myself, but that was made with the english piecing method over papers, here is the link to it if you want to have a wee squise.
http://www.quiltingboard.com/posts/list/1328.page