Why is it that the people least able to afford dental care have the worst dental hygiene?

I went over to r/povertyfinance out of curiosity this afternoon and at least half the posts are people complaining about the quotes they received from dentists for the extensive dental work they need.

The stories are literally all the same. "i grew up in a poor culture where dental hygiene wasn't stressed, and am paying the price years later".

My question is: if you are poor, and especially if you are poor with kids, why aren't you even MORE diligent about taking the 5 minutes in morning and at night to do what is necessary to prevent dental issues in your mouth and your children's mouth.

In other words, why is dental hygiene not stressed among the underclasses?

On the other hand, dental hygiene is best among those MOST able to afford expensive dental work

Bottom line, I don't get why something as simple and dirt cheap as brushing and flossing is stressed at different rates along cultural lines.

A lot of posters on here grew up in poverty or lower middle class, so I am curious if your poor grandparents/parents ever broached this topic.