Why was the canadian embassy involved in the rescue of american diplomats rather than the american embassy?

Because the six American diplomats were traveling with real Canadian passports and forged entry documents, easily made it through security at Tehran's Mehrabad International Airport.

The "Canadian Caper" was the popular name given to the joint covert rescue by the Canadian government and the Central Intelligence Agency (CIA) of six American diplomats who had evaded capture during the seizure of the United States embassy in Tehran, Iran, and taking of embassy personnel as hostages by Islamist students and militants on November 4, 1979.