Got absolutely nothing to do with throwing a knife, from google:

The 21-foot rule was developed by Lt. John Tueller, a firearms instructor with the Salt Lake City Police Department. Back in 1983, Tueller set up a drill where he placed a "suspect" armed with an edged weapon 20 or so feet away from an officer with a holstered sidearm.

The Tueller drill is more or less a requirement in defensive pistol training.