I think it's to do with what they are investigating and how far they need to insert the camera.
Enema for not far up the bum, mega-laxative for further up the Marmite motorway.
Just a thought though. Await Sqads answer :)
Perhaps it's to do with how well it's felt the patient would tolerate the bowel prep stuff, it's quite a large volume of fluid to get down and quite aggressive in how it works. If the patient is having stomach issues as well as bowel problems then it may be difficult for them to get and keep the prep stuff down.
And also perhaps due to the part of the bowel which is being investigated.
I had a sigmoidoscopy with no bowel prep or enema, something of a surprise!
Maybe laxatives are more dehydrating than enemas so not appropriate for some people who are already at risk of dehydration? Just a thought. I've had a colonoscopy before with a laxative, and for my sigmoidoscopies now I have no prep (thank goodness!)
I had the laxatives (hopefully never again!) for camera up (and down at the same time) and an enema for a sigmoidoscopy. I also had this vile drink (worse than barium, which is saying something) for a bowel MRI, I had trouble even getting it down as it just kept coming back up.
I do wish they could invent some nicer solutions but a necessary evil!