Not all competitors will have possessed such mags prior to 2013 (they weren't old enough, weren't into firearms before then, didn't get their current gun before then, whatever). Since not everyone can legally have 15+ round magazines it's a competitive equity problem. This was literally the exact reason why rule 3.3.1 was put in place. It probably would have been easier if USPSA had said 3.3.1 was applicable when it was announced that Nationals was being held in Colorado. I'm guessing that they didn't because that would probably dampen the enthusiasm some potential attendees would have about attending the match. Or maybe hold Prod/L10/Single Stack/Revolver Nationals at Cameo and the other divisions someplace else.
But the USPSA BoD seems to revel in making stupid decisions, so this is all rather unsurprising.
Why would IDPA care about >10 round mags?