Couple of reasons for using cast iron. The gray stuff is usually a lot
easier to machine than steel, it's somewhat softer so it doesn't wear
the spindle threads and has a somewhat more damping effect on
vibrations than steel does. If you ding it, it doesn't raise burrs,
either. Castings are usually cheaper to make out of iron than steel,
too. I've gotten disks of gray continuous cast from MSC to make
backplates for my 7x chucks, those are stud mounted. I've made
threaded backplates for a South Bend from hot-rolled flame-cut slugs
from a crane outfit, too. Cleaning off the flame-cut surface wasn't
fun.

Stan