Probably because they make it to use their pistol mags ? or it might be because they want to build one model that can sell in all 50 states.
Remember, Hi Point is appealing to the bottom of the economic pyramid by providing weapons for people who want to be safe, but are also worried about making the rent or groceries. H/P can save a few cents by not having to match magazines with destinations.
Also remember the SKS also holds 10 rounds, (granted they are more powerful ) and the Garand, (remember what Patton said, ) only holds 8.
What about after market mags ?
Hate to say it, but the mag issue is a plus for Glock. 33 Rounds may not work for CCW but .....
