There was a very interesting discussion recently on this topic. [1]
There are various hypothesis:
* changes to gut microbiome composition due to modern diets and overuse of antibiotics
* excessive cleanliness and hygiene. The "hygiene hypothesis" suggests that lack of exposure to diverse microbes leads to improperly trained immune systems
* increased exposure to chemicals, plastics, and pollutants that may disrupt immune function
* Cesarean sections and formula feeding rather than vaginal birth and breastfeeding