Definitely the pay.
The facility I worked at paid DOUBLE that rate that the local hospital paid their RN's, and it was practically unlimited overtime.
It was also both easy and challenging. Routine was easy, assessing and treating urgent or emergencies was challenging. Being absolutely in control and assertive at all times when facing down an IM is tough. facing down the doc when the IM's BP is 70/30 and the Doc says he's faking is tough.
(IM stayed 7 more hours!!!

and BP got down to 50/20 before they sent him out)
Fortunately for me, there was good rapport between the CO's and the Medical and we didn't try to do each other's work. Of course the occasional turf battle, but it was mostly very good.
However, it is a very isolated world and I wouldn't want to work only there. After a while you begin to lose/forget skills you need in acute care and I like working the floor, sooo.... that's why travel nursing exists. I can always go back.
I can't judge for all but I would suspect that a lot depends upon the facility as well. The state I'm in now one place has a bad rap and another everyone says "oooh you gotta go it's great!"