because people just starting out should focus on getting stronger in the basic lifts, and 5/3/1 provides a solid, idiot proof way of doing that.
Also 5/3/1 is so flexible it can be tailored to any goal. Bodybuilder types can use high rep accessory stuff and do all the pushdowns and lateral raises they like. Athletic types can programme in shit like box jumps, power cleans or whatever.
As long as you are doing the big, basic lifts and sticking to the % recommendations, you can do whatever the hell you like (as long as it doesn't decrease your performance on the basic lifts).
So getting you strong + easily tailorable to specific goals = one of the best programs ever written