It's popular because it's very simple and straightforward. As long as you follow the progression scheme outlined in 5/3/1 for your 4 main lifts, you're free to choose whatever assistance exercises you want. Also, it's easy to judge your progress on 5/3/1. You can just use Wendler's 1RM formula to compare one week to the next and one month to the next. As long as your calculated 1RM goes generally upwards, you're making progress.
"Generally people can't squat because they're lacking in the 'lower ab' area. As in they need to grow a set." - LtL
"No one ever got big or strong by lifting lightweights" - Carl1174
Follow my log here: Aurik's Back to Basics log
Valentin Konstantin Platz