He already did, in a way:
"Youâ??ll notice a lot of overlap with some of these exercises because weâ??re trying to do more with less. Thatâ??s training economy, a very good thing; better results with less time in the weight room."
It's just that the Wendler Six manage to complement the big four very well to make sure no muscle group is left out. Sure, you can do dumbbell presses instead of dips; you can do many things that will work. But if you have to pick only six assistance exercises, these are a very good choice.