Supporters of Creationism and Intelligent Design don't know the difference between these three. Occasionally you'll hear one mention something about Big Bang; but many of them just figure the Big Bang was step-one of Evolution. They are completely oblivious to the theory of Abiogenesis; let-alone the separation of the three.
I'll not claim to know since I'm not a creationist, and I try to limit my time around them; but I think I've got a pretty decent speculation.
First these are real, accepted, peer-reviewed, and constantly scrutinized theories - and we all know that creationists disregard all real science. Secondly, and I think more prevalently, in their hypothesis Yahweh did it all in one step. So in their mind, they figure since Yahweh did it all in one step (over six days), and since the opposition is just opposing them for the sake of going against Yahweh, the opposition must be saying its version took place in one step too.
I'd love to write a nice long article about the three, but really I'd just be paraphrasing and copying others' work. So how about this instead, here's my three favorite videos on the trio. If you know of other good ones, please share (especially if you think they are better). Arm yourself with knowledge! Make sure you at least know the basics of the three, and most importantly that they are three distinct theories.