Historian Ruth Sime has written about Meitner's and Frisch's omission. For one, the 1944 prize was a Nobel in chemistry; Meitner and Frisch worked out the theoretical interpretation of the physics of atomic fission. Hahn's Nobel was essentially for discovering nuclear fission via chemical analysis, which showed that barium resulted after uranium was bombarded with neutrons. Perhaps more importantly, Meitner was an Austrian jew, and when Hitler annexed Austria in 1938, she automatically became a German citizen. Being a jew, this was problematic, and so she was forced to flee to Stockholm. It was harder for her to continue the work on fission from abroad, especially given the decaying situation in Europe once the war started. Hahn consistently downplayed the role of physics in the discovery for the rest of his career. See Ruth Sime, "An Inconvenient History: the Nuclear-Fission Display in the Deutsches Museum," Physics in Perspective 12: 190-218.
As to Strassman, I'm not quite sure why he was left out; Hahn was an enormous figure in German science, though. And the Nobel prizes are enormously political in nature--see Robert Marc Friedman's The Politics of Excellence: Behind the Nobel Prize in Science (2001).