I'll tell you why. Because they have no respect for (among other things) entertainment value, or the very industry they represent (Ryan Drummond was willing to relocate to New York, but they blew him off anyway). They also assume that the viewers want everything to be American, and as such assume that Americans "don't want anything to do with Canada, eh?"
Still, since this is a third party series airing under contract, Nelvana probably won't let them change the voices, especially since Nelvana has had a U.S. presence for years despite operating out of Canada. Nelvana likely won't stand for anyone "breaking their backs".
They might do it anyway, but again, Nelvana is completely unlikely to let them.