Why do we rarely see Full-Backs anymore?

I’ve been watching the Steelers & NFL more religiously in recent years, and I can’t help but notice that the Full Back position seems to be an endangered species right now, but I’m not entirely sure why. I would have thought that with the resurgence of running games -/ the Brady-Manning-BigBen-Rogers era began to end, Full Backs would be more essential again.

Anybody know why more teams don’t use them very often / at all?