Why are more people deathly allergic to random ubiquitous things like peanuts than there used to be?
They aren't: you just notice it more because people randomly up and dying is less usual than it used to be.
They aren't: in the past they just weren't exposed as much because things in general were less ubiquitous.
Because improved medical treatment has reduced the selective pressure against stupidly overactive immune systems.
Because more random dubious crap is floating around (e.g., more virus strains due to increased travel) causing people's immune systems to be "on edge".
Because less random dubious crap is floating around (e.g., because the world is "too clean"), causing people's immune systems to get bored and obstreperous.
If I have missed an explanation, please explain in comment. If you wish there were checkboxes instead of radio buttons, prioritize or flip a goddamn coin, you squishy-headed fuck.