Some people are convinced that it's due to a lack of vitamin K2 in the western diet (not K1). I personally believe this too, at least as a contributing factor.

K2 is what pulls calcium from your blood into your bones (and teeth). It's mostly found in fermented foods (natto, kefir, cheese). Your body can make your own, but it's thought that it's not be enough to stop tooth decay and osteoporosis. It's my understanding that it's only been about 15 years since we have discovered that people might not be getting enough vitamin K2, so the science might not be settled yet, but there's evidence eg this [0] recent periodontitis study found that people with periodontitis had lower serum levels of vitamin K2.

Other vitamins and minerals that are often lacking that contribute are vitamins D and A, and magnesium. People often get insufficient amounts of vitamin D and magnesium in the modern day too.

[0] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10670967/

My teenage cavities stopped after 1) eating less sugar, and 2) switching to a non-fluoride toothpaste. Now that I'm in my 30s, I have had no cavity issues thus far, but I do have a lot of chipped teeth. I should try fluoride toothpaste again, in order to see 1) if it was the lack of sugar that reduced my cavities, and 2) potentially strengthen my teeth.

Why are teens eating so much sugar?