Because nobody will want to hear it. You can be NC and everyone winder why and hate you or you can be NC and have a massive row first and everyone go around telling everyone else what a total bitch you are and hating you more.
Telling him anything just means he'll seek to keep the abuse better hidden. It doesn't mean the child won't be at risk of abuse just because he knows her parents are onto him.
Even maintaining contact with someone having told them you suspect they're an abuser gives that abuser the information that there's very little you won't tolerate. If someone is going to tell or infer to someone that they're an abuser, the normal reaction would be to not want to see that abusive person. To continue doing so is telling them you're happy to break down (or fail to erect) your own normal usual sane and ordinary boundaries. It's basically announcing to the abuser that you're weak. How's that ever going to work out well for you?
There's only really three options.
Say nothing to nobody and just exit yourself from the family by going NC leaving them to wonder why.
Carry on like there's nothing wrong, continue to visit alone and make endless excuses why the rest of your nuclear family don't visit any more.
Let everyone know what you think (it will be everyone, the accused will defend themselves to everyone) and go NC.
I expect that with not being believed, because there's no hard evidence, most people couldn't be doing with the row. Children get raped and people still don't believe sometimes. The not being believed can add another layer of trauma too. It can be healthier for the victims sometimes to not to confide in other family members or confront the abuser.
People can't believe can they? If they do, they'll have to act on that knowledge, unless they're in favour of abuse. To carry on as usual like nothing has occurred would be them condoning the abuse (and enabling abuse if they're also providing the abuser with access to victims).
Easier to disbelieve and only lose one relationship, the accuser/victim, then they can all carry on as usual and not have their own extended family relationships/marriage (in the case of the abusers wife) blown apart. If you believed someone who told you your father/uncle/brother/ husband/friend was a paedophile, you'd not be happily going round to the paedophiles home next weekend for Sunday dinner with your DC, would you, and you'd not want to stay married to someone who is a paedophile either. It suits some people not to believe, it's convenient.