I had spoken to the folks at Tapco about that some years back before Remington bought them out and IIRC their reply was something on the line of extra spring tension and wall thickness at the feed lips causing a lot of breakage as well as the bulging Toad mentioned.
Also, making the feed lips thicker wasn't an option because it would change the dimensions and dynamics of the feed and give potential interference at the bolt. Thus, their conclusion was that 20 rounds for the SKS was optimal.
Also, if you compare it to their AK 30 round mags you will find the thickness of the nylon to be just a little fatter and that the magwell of the AK allows for it whereas the SKS does not.