Spoilers: It was because of anti-communist sentiments and policies.

Canada’s Immigration Department after the war determinedly and successfully prevented communists and other leftists from immigrating to Canada or even visiting the country.

...

My interpretation of the immigration files, along with cabinet discussions, is that the same anti-communist obsession that dominated immigration policies before the war and resulted in indifference, if not actual support, regarding fascists and Nazis who might be entering Canada, quickly re-established itself after the war.

Right-wing extremists, rather than being viewed as a security threat, were viewed as potential docile workers who would be on the “right side” as Canada made the fight against communism internally and worldwide the centre of its foreign policy. They would be bulwarks against Marxism and Marxists in Canada.

The government actively chose Nazis over communists, Jews, and people of colour, because Nazis would oppose communists, Jews, and people of colour.

I'm shocked. Shocked, I say.