I mean, the Queen could just as well have been a made up figure to you or me, given the vanishing possibility she would affect any of our lives directly. Yet after watching those stories about her life, the monarchy, it manipulates your neurons to actually have a person to mourn. Funny, isn't it? And the length of her life certainly gave enough material to feel some story.
I imagine that before QE2, much state/people mourning of the sovereign was just symbolic, and though perhaps somewhat heartfelt (I daresay, but more for loss of the symbol), not deep. For all their quirks and personality problems revealed to us on TV, it actually caused them to mean more to us.