Because it is dangerous to play softball or other sports in bad weather. Besides the danger of lightning strikes while you are standing out in an open field with a metal bat in your hand, the field will be slippery when wet. It is much easier to fall or twist your ankle. read more
If there is just a rain delay, the weather may clear, and the games can often proceed, if the field conditions are still acceptable. This gets the games played and does not mess up the schedule going forward. read more
Rain delays can last hours, which wreaks havoc on pitching staffs. Each team is kept up to date on when play may resume so that each pitcher can warm-up in the bullpen. Once five complete innings have been played, the game becomes official; whichever team is leading after the fifth wins if the game is called due to weather. read more
In cricket or baseball, the fielding team will have an enormous advantage in moderate to heavy rain given the different visibility and frictional conditions of the ball and bat, and there is no guarantee that the conditions will be the same for their innings (in cricket, the call of who gets to bat first is already an extremely important factor, even with no rain play). read more