Pine tree monoculture has played a large part I would reckon.
And not just planting pine trees, but the way they are planted. We had good quail hunting in Perry county until the early 80s. And we were hunting on clear-cut land that had already been replanted, but the key was that it was site prepped with dozers instead of herbicides. The quail didn't disappear until they started using herbicides.
I am not making a wild claim that herbicides killed the quail; they just didn't regenerate weeds and grass the way mechanical disturbance of the soil did.
I don't think it's any great mystery what happened to the quail - it is entirely due to habitat changes. People with good habitat still have them. You won't get them back on a few hundred well managed acres either; it takes big sections of land that are at least decent for them.