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.
Punish people for things they have no control over. You’re a smart one.