It was cheaper

Slate.com article from 2006

Even though Canada has spectacular settings, it's not the production values that film producers go there to find. The lure is, in a single word, money. In Southern California, the studios have highly efficient soundstages and an abundance of skilled technicians, but the unions' work rules make it extremely expensive to shoot exteriors. For example, a production can shoot for only 14 hours a day with normal overtime and then must pay double time. It also must employ redundant Teamster drivers to chauffeur actors to and from locations (even if they have their own drivers). These costs run even higher for independent producers—about 9 percent on average—who are not part of the National Term Agreement that the studios have with the unions. As a result, the indies need Canada—or another deeply discounted country.

In Canada, producers still have unionized labor to contend with, but they get a huge discount—in the late 1990s, it was as high as 35 percent—by paying labor in Canadian dollars. On top of that, the Canadian Federal Government provides foreign producers with a subsidy called the Film Production Services Tax Credit, which now equals 16 percent of the Canadian labor costs. (It was recently raised from 11 percent to offset a rise in the Canadian loonie against the American greenback.) Also, British Columbia offers an additional 18 percent rebate on labor from that province. Finally, there is a 20 percent break on digital effects, if they are done in Canada.