Well, check out our own world history. WWII is talked about a lot more than WWI. Why? Because it was many times bigger and much, much more destructive than WWI.
Skyrim talks about the Oblivion Crisis because there were Oblivion gates opening up in Skyrim as well. The book just explains how all that managed to stop.
It takes about the Great War, which pretty much plunged all of Tamriel straight into Hell.
No one talks about the floating city thing because
a ) Some Nords don't want to believe that nonsense that a city can float in the air. Because it didn't reach them, it means little to them.
b ) The Great War that ended only 30 years prior to Skyrim is still fresh in everyone's memories. Oh, yeah, and dragons are coming back and there's a civil war raging.
WWI impact is far greater than WWII, WWI actually set teh world up for WWII.
IN the same sense the Oblivion Crisis has spawned everything up till now. The Thalmor coups, the rebellions against the empire etc.. is all because of the Oblivion Crisis. Umbriel hasn't done much, it was a small scar compared to the impact, and i thsi way is not a great event.
I'm a little upset we only see small snippets of the Great War.