Marley was dead to begin with...

Why does the narrator make such a point of Marley's being dead?

Because the story all hinges on ghosts...Marley and the three ghosts that come to visit. If one didn't believe in any of them there would have not been a story or a change in Scrooge's life.
-Bookworm

I think it is to make it more spectacular when he comes to Scrooge.
-Annegirl