Because patriarchy, more than anything else, is the sum total of conditions and assumptions and practices according to which men are taken seriously, given consideration, listened to, given grace, given the benefit of a doubt, etc, while all the stories and claims and perspectives and voices of women and children are viewed with skepticism and suspicion and caution. "Maybe she's just saying this because she's trying to ruin the life of this poor man." "Maybe this kid is exaggerating, because they don't understand the consequences of what they say..."
Excusing and accepting abuse from men isn't unconditional in patriarchy. It's conditional. But abusers know how to meet the conditions. They thrive in patriarchal contexts, they get away with their menacing craft, they hide their malignant narcissism, because they know how to say and do the right things to convince and placate all the people around them. Whether or not they are grooming their victims, they are always grooming community members. They know how to weaponize patriarchy's reflexive distrust of women and children to their benefit. This guy played this system for years, and when his family somehow found a toe hold that would enable them to actually use the system to escape, he annihilated them. And he'd been showing us all his willingness to do this for years before he actually did it.