Because look at it. What looks cooler than a portrait of the devil from one of the most (theoretically) evil books ever written?
Not evil, its just the regular thing, just bigger, with the picture of the devil and a few things about exorcism or something envolving rituals.
Point is its just a bigger bible with a pic of the devil, not exactly the evilist book ever.
The Devil Bible is just a regular boring bible you know. It's just named that because of that pic. Wikipedia has an article on it
This. Yeah saw it in a documentory, it was called the codex gigas or something.