Question: Why were they made to only take 12rd Mags?
Originally, 20 round magazines were standard, but in practice they were weak and prone to damage. Coupled with an already finicky firearm it generated a great deal of problems. The 12 round magazine was single column, single feed to increase reliability. Perhaps if Harrington & Richardson wasn't so dead set on competing with the Thompson, they would have designed it to use the Thompson's double column, double feed design.