As hard as it's been raining all morning on the North Side, they could have called this one at 10 am. As I understand it, the home team and MLB are in control of the game until game time and apparently they weren't going to call it early. Cubs actually got some concession revenue before the game that never was. And now they have to make good on the single game tickets sold. The Cubs don't want to cut their own cash flow if they can help it, so they rarely call games early.
At the time at which the umpires were given control 20 minutes before game time, they called it immediately. The outfield would have been unplayable.