Capped roofing nails... plastic vs square metal vs round metal

big box stores in socal only sell the square caps, with the nail head protruding.
and the plastic cap ones.

How come socal stores don’t sell the metal round caps anymore?

Not sure but why are you buying from big box stores? I get it for a homeowner but if you’re a contractor you would ask your local supplier that question and most likely have your question answered there rather than here.

I have never seen steel cap nails anywhere except a roofing/siding supplier.

Why do you need steel?