Because we are a nation whose collective memory is dominated by Baby Boomers who can't even remember the Truman presidency. LBJ, on the other hand, is the guy who almost got me sent to Nam. This also had something to do with it:
I think there's a lot of truth in this. I recall growing up in the 60's when Truman was not at all well regarded. Korea was a mess in foreign policy and there were a number of scandals among senior administration officials, which led Ike to campaign on a platform to clean up the "mess in Washington." History now looks more at the term he inherited from FDR with high marks and downplays the second term which was weak by most any comparison.
I'd hope in time that people look more at The Great Society--whose programs have arguably helped many more than Vietnam killed--as LBJ's legacy. The Great Society did tons of positive things which I'd argue outweigh the bad of Vietnam.
Truman's outstanding legacy today is being there to end WWII and the Marshall Plan, the latter of which he deserves total credit for.
People don't really remember that he was considered maligned in his own time. Korea was the Vietnam of the '50s in some ways, with less television and (I'd argue) a less self centered generation of age at the time.
I mean consider this: Nixon is almost totally (at least members of my generation, I'm 22) rehabilitated. A lot of politically oriented people my age think Nixon was a great President. I'd argue his crimes with Watergate and underming the peace process in '68 were much worse than Vietnam.
While Vietnam was a misguided war, it was not the literal constitutional crisis that Watergate was. Vietnam was borne out of ignorance, misguided ideals, and a sense that the US was (or should be) unstoppable. Watergate was borne out of paranoia, self-centeredness and Nixon's desperation to get re-elected.
