Anyone interested in reading a book on this topic can pick up “Diabetes Rising” by Dan Hurley. He is a T1 journalist who was aware of the increase in T1 and wrote this book to investigate the topic. He mentions both improved diagnosis, and lower natural selection losses (I like that euphemism for keeping us alive from Michel’s link!), but says that those are not sufficient to explain the extent of the increase. (Also note that in Michel’s link it says “Reduced natural selection may have contributed to the increasing T1D prevalence worldwide.” - not that it explains the whole increase).
He has a chapter each for what he considers the best contenders - 1. population weight gain; 2. babies consuming cow’s milk instead of human milk; 3. persistent organic pollutants in the environment; 4. too little sun and vitamin D; 5. babies not exposed to enough environmental dirt setting up later autoimmune reaction. Personally I found the last one to be the most plausible, though clearly none of them could explain all the cases of T1. And none of them has enough evidence to be clearly the smoking gun (or even the favorite). So maybe it is some combination of these causes, or even some other cause not considered.
So it is an interesting book, but unfortunately it doesn’t answer the question of why. Unfortunately, it seems that we just don’t have that answer yet.