As posted before, the problem is a fed kat has little counter-play, and the counter-play that is there puts a lot of pressure on people to play at a level most are just not at.
The first reason being, of course, the resets. The second reason being Woogles, which was not balanced with this mode in mind.
But the main reason: Instants.
Instant casts, instant resets, instant death.
If she can kill you before the casting animation of even your first spell finishes, or if you're not smart casting and you actually have to take the split millisecond to target her after she jumps out of a bush or into a hectic 12 player teamfight, you or whoever her first kill reset starter target is have not just little, but zero counter play against her.
Oh, and the final reason being it's a featured game mode, so MMR and matching is way off. You can only do so much to keep a kat from feeding off any one of 6 random people in that situation.
Then you have a fed kat, a champion that is rewarded for kills more so than any other champion in the game. One kill from a kat can mean you lose the entire 6v6. Because it's going to happen in the blink of an eye if the Katarina has even a mediocre amount of skill.