At some stadiums, photographers are restricted to certain parts of the field, making it even more important to have several shooters. The home team issues a limited of credentials for a particular game. Credentials are issued to photographers working for news organizations who have requested them and are known to the football club to be legitimate publications. It's all but impossible for an individual to obtain a sideline photo pass to a professional sporting event. This isn't snobbery, just the club's way of making sure that only legitimate news-gathering organizations have.
Space is limited and sidelines can get very crowded.
Each photographer has a different style and technique. Some like to kneel when shooting. It makes action photos look a little cleaner and enhances pictures in which the players are airborne. Most kneeling shooters wear knee pads for obvious reasons. Camera bags are completely impractical and sometimes verboten on sidelines, so equipment and film is carried either in fanny packs or in a photo vest.