I found the following article interesting, regarding genetic risks associated with T1D.
Genetic Risk T1D
That may lead to the conclusion that people diagnosed 100 years ago, didn't live long enough to have children, thus lower numbers of T1Ds. As we started using insulin in 1920's, T1Ds started to live longer, and to have children. For many years, women with T1D were advised to not have children, which has changed over the years now that newer insulins, insulin pumps, CGMS, etc, allow tighter control.
I have no history in my family of T1D, and many siblings. However, I know that due to breast infection, my mother did stop breast feeding me early. The article mentions some correlation with bread-fed children having lower risk, possibly due to improved development of their immune system. Don't know the statistics, but I think for a while, breast feeding was a downward trend, maybe resulting in higher trend of T1D.