Pumps are slow to reload and subject to operator short stroking under stress.
The Saiga-12 feeds from a 5, 8, or soon 10 round detachable box magazine. And possibly a 20 round "circular mass feeding device" in the future. Even with the kludgy Knoxx tack-on snail mag for the pumps, reloading a pump under stress takes considerable dexterity.