Mybat: it’s the volume. Back in the heim the shochtim were shechting these chickens anyway. Now you have a whole kaparos industry set up that hasn’t had all the kinks ironed out.
What happened in between? All those years in the US once shechita was institutionalized already, what was kaparos with chicken like? Were there just fewer people interested so it could be run in a more orderly acceptable way? How to get back there? With a lot of input from a responsible rabbinic committee and added expense that no one should complain about, I’d think.