I agree, I’m just answering the why question. Free software licenses don’t have non-commercial clauses and they want an NC clause.

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.

There is a more performant C++ implementation but it’s been a long while since I’ve used either it or the java implementation. Worth checking out.