There is no _one_ reason, and I don't present that particular phenomenon as a causal factor but as a symptom of the greater problem - which certainly includes poverty but is even more closely aligned to the opioid epidemic.
All this is in the context of asking parents to provide their children the guidance required to avoid child-inappropriate content.
My point stands: a large and growing contingent of parents lack the stability/ability/support required to even keep their children's behavior within acceptable boundaries. It's a fool's errand to think keeping kids away from bad actors on the internet can be added to their plate.
It's far more likely that lower income is the reason for poor parenting than "mom has an onlyfans".