<font face="Verdana, Arial, Helvetica, sans-serif" size="2">Now, I know it's low season, but found it a little odd. If memory serves, I don't remember seeing fares lower to U.S. midwest than Ontario/Central Canada.</font>
I think it's a result of US carriers refusing to fly into smaller centers, preferring to only cream off the most profitable routes. After all DSM (Des Moines, Iowa) only has a population similar to Windsor, ON, with a city population of about 200,000 and a catchment area about twice that.
To serve that population, between 8am and midnight, there are only about 70 inbound flights, on 18 different carriers. Even if you include all the codeshare flight numbers it's still only about 116 "flights" a day.