Ideally you want to use QMK's tri_layer feature as I outlined in this comment.
Also you can just use the _______
keycode instead of restating MO(...)
on every layer as that is the an alias for KC_TRANSPARENT
meaning it maintains the function of the layer below.
I presume the reason they didn't use GPL3 is because they wanted the attribution and non-commercial clauses offered by CC-BY-NC.
Not suggesting that they should not prefer to drop those clauses in favour of a copyleft free software licence. but you asked "why not" and losing those clauses is clearly an obvious candidate for why they might not want to.