Truman ran at a time were the Democratic Party was seemingly splitting itself apart, and he had opposition from the party’s progressive and conservative factions in the form of two third party rivals: Henry A. Wallace of the Progressive Party and Strom Thurmond of the States Rights Party. It was expected that the moderate Truman would divide the Democratic base allowing Dewey to win states he might unlikely win otherwise, similar to the 1912 election in which Theodore Roosevelt and William Taft spilt resulted in President Wilson. In theory this would work, but both Wallace and Thurmond came off as extremists making Truman look better in comparison.

This confidence in victory meant that Dewey put very little effort into his campaign, usually avoiding partisan politics and personal attacks. Dewey was also infamously socially awkward and quite bad at giving rousing speeches. To quote a 1948 op-ed from The Courier-Journal:

> "No presidential candidate in the future will be so inept that four of his major speeches can be boiled down to these historic four sentences: Agriculture is important. Our rivers are full of fish. You cannot have freedom without liberty. Our future lies ahead."

In addition Dewey was a “Rockefeller Republican” a Republican liberal on social issues. His platform included expanded Social Security, funding for public housing, and civil rights legislation. This put him at odds with the conservative factions of the GOP, including his arch-rival conservative Senator Robert Taft of Ohio.

In contrast Truman personally attacked Dewey and other prominent members of the GOP. It was easy to characterize Dewey as a frozen leader and that the Republican lead Congress would be obstructionist under either candidate. Truman’s lively rallies and media appearances made him feel an engaged leader.

The three states most critical states to the election results were California, Illinois, and Ohio, all won by Truman by narrow margins and together worth 78 electoral votes These states, were orginally predicted for Dewey. The Midwest which Dewey had won in his 1944 campaign against Franklin Roosevelt as well went to Truman.

As for why the polls predicted a Dewey victory, in addition to the divided Democratic Party, pollsters relied too much upon modern technology largely polling via telephone. Truman stealing the Midwest rural vote from Dewey was not calculated due to many not owning home phones. In addition Truman performed better among white southerners against Thurmond than some guessed, alongside traditional Democrat demographics like Catholics and Jews.

Tl;dr:

Third Party candidates underperformed to expectations

Wallace made Truman look like a conservative, Thurmond made Truman look like a liberal, making him more palpable to Democrats as a whole.

Republicans put forward a candidate that was too stiff

GOP congress was easily characterized as obstructionist and was unpopular with the public

Truman was willing to say controversial things, go for personal attacks, and address issues Dewy wasn’t

Demographics such as Catholics, Jews, and rural whites were underrepresented in new polling tech