Ruger always uses proprietary mags.
I'd have bought a Mini-14 if the used AR mags, and their .45 mags are virtually identical to 1911 mags but just different enough so they won't interchange.
My guess is they do that so people won't buy cheapie mags and blame the guns when they choke because of lousy mags, or so they can get the money when people buy spare mags.