Purely because of popularity. Breeding BPs are in right now. Don't even get me started how many people breed and/or sell these snakes that have no clue what they are doing and even more people buy them having no clue what they are doing.
Sad story... went to an expo this past weekend and as I was walking in some young kid was walking out with two green iguanas in his hands and someone else was walking behind him with a bird cage that they had bought as an enclosure (hint that is a really horrible enclosure for them). I am so glad larger snakes are still not as common as ball pythons. The amount of adult large snakes abandoned is enough to make you depressed.