This has been submitted before, but the answer is not correct. Richie, Eddie, Beverly, Ben and Bill don't remember anything about each other or the events in Derry. For example Bev's husband reads one of Bill's books and she doesn't recognise the name, and Bill doesn't know who the architect Ben Hanscome is. Why is it that Stan not only recognises Bill's name and remembers him, but also remembers the Turtle? He remembers all of this long before Mike calls.
[The longer they're out of Dairy the less they remember about the specific incidents. That doesn't mean they don't remember any of it, just that their memory is dim and they've forgotten large chunks of it. They still remember parts of it in their dreams too.]
Answered by MovieGuy