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.
Ricardo was testing in production
I know this doesn't answer the question but I want to offer some advice instead.
In my opinion just don't. If the company want you to have access to emails on the go then they should give you a company phone. If they don't, why are you trying to? Don't put work things on your personal phone.
The main difference that has me using LibreTube rather than NewPipe is because my subscriptions are on my piped account so they are synced between phone and desktop (browser). Piped is built on top of NewPipe's extractor library anyway.
I agree, I'm just answering the why question. Free software licenses don't have non-commercial clauses and they want an NC clause.