Partly habit on my part. I've only ever kept males (a collector, not a breeder) and I have no issues keeping a BP up to 1800 ish grams in an Iris sweaterbox. Not a lot of males get much bigger than that unless you over feed them. (yes, I know there are exceptions out there). Some of the bigger girls, however, would probably need the little more space found in a CB-70.
I don't think there's anything wrong with keeping males in a CB-70, and to be honest, might do so myself in the future. It's just too bad the sweaterbox racks and the CB-70 racks don't stack together.