It's not just one thing. Most of those things on their own won't even kill them. For example, Varroa mites will kill an already weakened hive, but not a healthy one.

Lawns absolutely contribute to poor nutrition, due to habitat loss. Same with all the mowed grass we have everywhere in suburbia. Monocropped agriculture does as well, because bees do best with a variety of flowers.

I've let the back part of my property grow wild the past couple years, and it's currently filled with a ton of goldenrod, chicory, and a bunch of other random flowers. You would not beleive the number of honeybees I've seen back there at once, or how loud the buzz was.

Similarly, there's a reason I see a ton of fireflys in my yard, but I see almost none in my neighbors yards. It's because they're well- manicured green wastelands