The reason is that OSFI placed a "stress test" on insured mortgages a while back. What happened next is that debt-loving Canadians decided to obtain their 20% down payment by any means necessary to avoid the stress test (and CMHC extra insurance cost).

People ended up going to fishy secondary lenders (paid higher rates), then brought the cash to the bank for their down payment. 

At the end of the day, CMHC insured mortgages fell by -40% in a short period (but sales did not). Banks ended up taking all of that risk on many falsified(fraud) financial backing.

OSFI needed to step in to shock people back in line and protect its true client, the banks...

Party is over.