Pelican pretty much nailed it. It has to do with waterfowl populations and hunter density. Generally, as you move west seasons become more liberal. This is usually a Flyway wide regulation. However, there can be differences within the Flyway (High Plains mallards in the Central Flyway).
The argument heard about seeing more mallards after the season is generally because those birds understand they are no longer being hunted. It’s not because there are new birds arriving. If you looking at hunter participation particularly in Northern states, hunter participation drops as the season goes on. So taking days from the early season to move later takes away hunter opportunity.