See "The Mighty Eighth(Air Force)" by Gerald Astor. He explains it is mainly about air crew losses. As losses decrease so a tour increases. In 1942-43 only one third of crews finished a tour. By 1944 its two thirds so the requirement goes up, first from 25 to 35 missions. In 1945 four fifths finished their tour of operations.
Jimmy Doolittle made the decision to raise the number of missions from 25 to 30 because of the high number of bomber crews that were being lost in the early stages of the air war in Europe. Ditto raising it from 30 to 35. I'm not sure of the rationale for the Pacific Theater having a 50-mission limit, although I'd venture a guess that the fighter and flak opposition were not as heavy as in Europe.
- the failure of the combined bomber offensive in 1943 [ie. aircrew losses, see "Operation Point Blank: A tale of Bombers and Fighters," by William Emerson, Harmon Lecture series 1959-1987. USAF Academy and Office of USAF History, 1988.]
-combined with the overall manpower shortage in the US manpower pool. See "The 90 Division Gamble" by Maurice Matloff from _Command Decisions_ Center For Military History, Wash DC, 1984.
Another useful article might be D. Giangreco's "Spinning the Casualty Estimates" from the December 2004 issue of _Passport_. This article highlights the manpower crunch from a different perspective.