I don't think MLB really had much of a choice. My guess is they probably wanted to see how the top and heart of the Rays order could do in the Top of the 6th.
Things really don't look good for Tuesday either and it is forecast to even snow a little on Wednesday.
10 Day Weather Forecast for Philadelphia, PA - weather.com
MLB rules technically allow them to move this one to Florida, but my guess is they'll just ride it out through Wednesday. Even if they get this thing in by Thursday, they still have Friday-Saturday or Saturday-Sunday to wrap it up (if necessary -- though I'll be pissed if it screws up Fox Sunday's animation block).