why a soccer ball that is quickly rolling across a grass field slows and finally stops rolling

friction accord while the ball was rolling in the grass and slowed it down until it finally came to a stop

This is because of friction. If you want to add extra detail you can say that this ties in with Newton's First Law of Motion, which states that an object moving at a constant speed will stay moving at a constant speed unless there is an opposite force, and in this case, the opposite force against the ball is friction.