Chick is probably right on this one. Bodybuilders have been using very high dosages of anabolics for decades now, but the kidney problems are more recent.
I'm not claiming steroids don't cause increased blood pressure which is harsh on the kidneys, but their role alone pales in comparison to the combination of steroids, the rigours of contest prep (diuretics, extreme dehydration, cutting drugs) and abusing pain meds.
Flex Wheeler himself said in an interview many years ago that it was the diuretics that were harshest, sending him many times in pain and agony to the emergency room. The crazy conditioning demanded by judges, and the willingness of competitors to go along with them, are to blame for this.