Because that's the experience everywhere, and Statistics Canada's own planning assumption is a substantial—perhaps overstated, but substantial—deterioration of response rate. The historical response rate to the long form was 94%. The current planning assumption of Statistics Canada is 50%. That's a huge deterioration of response rate. Even if they exceed it and they get 70%, that's still a huge deterioration of response rate from 94%.
On March 8th, 2011. See this statement in context.