Because the Obamas would have been invited as a 'political guest'. WK wedding was not a state occasion. No heads of state were invited, other then governor generals of the commonwealth, and royals. Same goes for Andrew, Anne and Edward's weddings.
If the Trudeaus are invited it will be INSPITE of the fact they are a PM and his wife. They will be invited as personal guests, as friends of the bride. Its possible to avoid the 'political angle' Sophie would come by herself. Sophie and Meghna's other friend Jessica Mulroney are friends, so she could accompany the Mulroneys over.
I don't get your comment about Autumn and social status. Am I missing something???? Her father worked in business (not sure he is any more successful or wealthy than Thomas Markle would be working in show business), and her stepfather was a pilot. Autumn too was an actress. She did acting, bar tending and show room modeling in her college days. The only thing really different is she wasn't divorced, and she moved on to another career.
But like Meghan, Autumn was never going to be the future queen. Certainly less expectations.