I think it has a lot to do with their temperament. The puppy dog tame aspect of most ball pythons is probably a big plus with beginners.

I have no experience keeping other reptiles besides bps, but as a beginner it was definitely a challenge getting things right. However I think anyone that is a person truly "responsible" about keeping a live animal could do what they needed to without much difficulty. The main problem I see is the availability of proper information in places like petstores. People often invest a good amount of money based on the wrong advice from pet store people only to have us tell them they need x,y,z.

I think thats the hardest part of bps as a beginner all the conflicting information you receive and somehow being able to wade through it to come out with your own way of doing things that work.

Edit: I mean once you get the temps and humidity dialed in its pretty much easy.