Plentiful nutrition which has led to 90% of the population of the USA being overweight, obese, or overfat is another potential culprit. Women suffering from anorexia developed fewer tumors. Similar experiments on lab animals in a controlled setting have the same result. https://www.ncbi.nlm.nih.gov/pubmed/11246846 I believe there are other changing risk factors in behavior as well, for instance women who have never had a child are more likely to develop breast cancer and fertility rates have dropped dramatically over the last 100 years.
Cell phones have only been widely deployed in the last two decades, and I don’t think those trends in cancer rates you’re referencing correlate very well to cellular deployment.
I’d still be curious to see more research happening in the field of millimeter waves, personally I don’t see this technology as very useful right now either compared to traditional cellular due to its lack of penetration.