Nobody notices the beautiful wallpaper after a while because because they become accustomed to its presence, and it fades into the background of their daily routine. It becomes a part of the scenery, and unless someone draws their attention to it, it may go unnoticed despite its beauty.
The same principle applies to a lot of beautiful things that surround people all the time. Once they become accustomed to its presence, then they take it for granted until a stranger brings it to their attention once more.
