The gimmick of Thor turning back into Donald Blake after 60 seconds without Mjolnir was never utilized well in the early comics, rarely mentioned later, and then discarded. It was always cheap drama like Spider-Man running out of webfluid. The actual quality stories by Stan and Jack were almost always with Thor interacting with the other gods. In fact the basis for both the film and loosely for Ultimate Thor is the story from Thor issue #145, "Abandoned on Earth!" The film needed to be about Thor as Thor. Amnesia has been used too often in other films and the mystery of Donald Blake not actually having a past would not have wowed modern audiences and would have distracted from the plot rather than enhance it. There have been roughly four or five different eras in the comic of Thor as a character, and the film masterfully blends them.