FWIW, I used to have more free time and was fairly active in the original IDPA email list way back when (my IDPA # is 1422 for what it's worth.).. I eventually gave up that list (yahoo groups) because I don't have time for the 100 plus daily emails it generated.
I probably haven't kept current with the lastest rules, but while we couldn't shoot, say 6" revolvers, in state or national matches, shooting them in local level matches (your regular club) was fine.
Without meaning to rile anyone up, I argued for 6" revolvers quite a bit before I learned the rules were already made and it'd take at least two years to change them. It has always been difficult for me to argue against success, and there are about a gazillion or so 6" .357's out there that seem to work pretty dog gone well. I have never preferred a long 6" revolver for CCW, but I will not argue aganst someone who does.
So. The powers that be, or one of them anyway, gave an explanation I don't quite agree with, but cannot argue against. The 2" J frame is probably even more successful than the 6"ers. And the reason (for banning 6"ers) given on that list to me was that no one would compete with their snub J frames if the 6"ers were allowed. The J frames just would not be competitive, and so no one would shoot them.
Well, I can understand how that would happen, but I still think 6"ers should be allowed. It's real life, and I know my J frame performance is no where near as good as my 4" midframe performance.
Of course, lots of folks don't quite always see things "my" way.
And my Hi Power is so much blasted fun that I still haven't shot IDPA with my snub J frame.