Praxis. It was destroyed earlier in the Kelvin Timeline as compared to the Prime Timeline. You can see the remnants of Praxis when they’re flying into Qo’nos. There’s a comic book prequel for Into Darkness that shows Khan going on a mission to Praxis to sabotage and detonate the moon.

The destruction is recent and heavily impacted the atmosphere of Qo’nos in both the Prime Timeline and the Kelvin Timeline. Praxis was also originally based off of the Chernobyl disaster so reflecting the destruction as a nuclear winter on the ground feels pretty appropriate to continuing that idea of Chernobyl and showing what it looks like on the ground a little while after Praxis. Then there’s the fact that they decided to reflect the Klingons in this time period as being heavily industrialized with a thick layer of pollution covering the planet.

Gotta say though, not sure how I feel about these comments just being dismissive immediately without even looking into the answer. Merely insulting it and saying it is undercooked or that it takes/doesn’t provide adds nothing to the discussion or theory crafting. The whole point of discussions like this is to come up with a potential answer, not to throw up your hands and say “They’re dumb lol”.