Because that's just how it is!
Actually, it was decided that there would be three legs in the relay because:
- there would be a relatively long (meaningful?) distance for each leg
- three legs would not overly clutter the course with too many exchange zones (and the additional cars, etc. that accompany them)
- more legs would create data-handling problems
- more legs would increase the logistics involved in timing the event by adding more exchange zones (and more timing hardware, volunteers, chute materials, etc.)
The first relay was held in 1991. It was created by Paul Beberg, who was the UAF Cross Country Skiing and Running Coach at the time, and was the Equinox Marathon Director.