Complex sentence: Until 1998, O'Hare was also the world's busiest airport by the number of passengers; it was surpassed mainly due to limits the federal government imposed on the airport to reduce flight delays.
Split into simpler sentences: Until 1998, O'Hare was also the world's busiest airport in number of passengers. It was surpassed mainly due to limits the federal government imposed on the airport to reduce flight delays.