I'm sure it has mostly to do with engaging multiple targets. It goes without saying that if you were faced with multiple tangos, you would want the most amount of rounds in your gun. While most "reviews" will state low magazine capacity as a caveat of those two platforms, it is generally included because the rifles are just really too dang heavy for any serious social work considering the light and high capacity rifles available today. I don't see any problem using an SKS or a Garand for the job. They are great rifles and few can deny that. They are, however, extremely heavy.
"Shut up, crime!"