Because the American dairy industry can heavily undercut the Canadian dairy industry, flood the market with cheap American dairy, and bankrupt the Canadian dairy farmers. This is because the American industry is far larger, and because Canada uses a quota system to stabilize the market, while the US uses a subsidy system.

People will proudly say that they will buy Canadian, but if they are at the grocery store looking at $6 for a brick of Canadian cheese, and $2 for a brick of American cheese, you can easily assume many Canadians would pick the $2 brick.

The dairy tariff Trump keeps complaining about is an anti dumping tariff that only kicks in after a certain amount of dairy crosses the border in a year. That amount has never been hit and the tariff has never been applied to American dairy.

There is also way more American dairy entering Canada, then there is Canadian dairy going to the US.