The 1948 (election) link to wikipedia states:"The election is considered to be the greatest election upset in American history.1" with no less than three sources.
One possible reason might have been the Congressional elections of 1946, in which the Republicans brought about a swing of 18 states, five in the West, five in the Northeast, and eight in the heartland. Of those, Truman went on to win the five western states, Dewey the five Northeastern states, and Truman won seven of eight in the heartland.
One factor in Truman's favor was that he came from Missouri, a heartland state, while Dewey came from New York. That gave Truman an advantage in the competition for Illinois, Ohio, Wisconsin, and of course, Missouri. On the other hand, Eisenhower, from Kansas, in 1952, won all but nine states, (the four Strom Thurmond states plus five other southern states).
Truman won the 1948 popular vote convincingly, 49.6% to Dewey's 45.1%, or four and half percentage points. What is noteworthy was that Truman won three big states, California, Illinois, and Ohio by less than one percentage point each. Swing the vote total by one percentage point in each of those three states (or even across the board), and Dewey would have won the electoral college while losing the popular vote.
Did people at the time make the argument that the electoral college "math" stacked up in favor of Dewey, even though Truman might win the popular vote? My sense is that this was not the case, and Truman was expected to lose the popular vote also. Why might that be?