Well, we know that Roland and his friends definitely passed through the world which was ravaged by the Super flu in
The Stand 
Perhaps, after writing his telephone book-sized novel about one apocalyptic setting in
The Stand that shifted through a whole cast of characters and showed the wide scale devastation caused, in
Cell he decided to put the blinkers on and concentrate only on a small group. I think it makes it a little more intense to follow just a handful of characters as they battle to survive. And perhaps people would have really been quick to write
Cell off as merely a copy of
The Stand if he'd followed the same formula and shown the disaster worldwide.
As to the loose ends and not finding out about the Pulse in
Cell -- I admit this is slightly frustrating, we do like some conclusion in novels. But I didn't mind all that much because, as I'm currently ranting about in a journal for my creative writing class, I think one thing that King does well, despite writing about supernatural, horrifying, inexplicable events, he's very good at the verisimilitude; and how likely is it that our small group of survivors, while battling just to stay alive, really, should stumble across the source of the Pulse? I think, without some huge coincidence, it wouldn't have really been feasible. I don't think any of the characters in
Cell were out to save the world (granted, they took out a large group of the phone crazies, but they were an immediate danger); indeed, for Clay, the driving force seems to be finding and saving his son.