On any given day, there are over 25,000 commercial airline flights and the concern of every airline and government employee is that every one of those airplanes gets to its destination safely. For 55 years, the U.S. government has been in charge of tracking flights to make sure they stay far enough apart from each other to be safe in the sky. While the sky looks limitless to you, an air traffic controller or pilot sees a set of highways in the air.
Bad weather can block the skies just like flooding can block a highway.
When that happens, controllers have less space in which to move airplanes around and still maintain separation between them. The only way controllers have then to maintain separation is to meter the number of airplanes allowed to fly in a particular section of airspace.