Why do mountains disappear into the distance?

Traveling south on CA 99 we noticed that atmospheric conditions were exceptionally clear and played a little game attempting to determine how far away we could identify Shasta. As we got further and further away it got smaller, greyer, and harder to locate. I took to using these closer (green) low hills that were nearly aligned with it to locate it. As expected, the low hills went down below the horizon.

Yet proceeding south I managed to reacquire it (partially using Lassen as a reference point), and manage to mostly hold it for quite some time. As we were coming up on Chico, it started to get close to disappearing below the horizon. But that never happened. Just north of Chico, Shasta disappeared into blue sky while I was looking right at it. It did not go below the horizon. It was not occluded by any cloud or smoke or smog, nor by trees for that matter. There was no indication of an atmospheric disturbance. I didn’t lose its location and fail to require it.

So why might Shasta have disappeared? What limit did I exceed that I could no longer see it?

(safety note: I was not the one driving, so I could indeed turn and look straight backwards for minutes at a time)