Because typically what happens is someone holds a ticket to Nats vs Braves, and it's postponed, the game they get to go to is also Nats vs Braves is because that game is scheduled at a later date. In this case. People with a ticket for Monday's game will have tickets for Nats vs Braves. Nats and Braves will still play. There is now, not a future game for them to go to because that game will have been played on April 5th. And they might not want to see the Mets.
Ok I get it. Makes sense.
