Anyway, I suspect part of the answer was that Stevenson was kind of an accidental governor. He was elected only once in 1948 defeating a Republican incumbent who had a huge backlash to him over his handling of this. He honestly probably would've lost if he had ran for reelection in 1952 anyway.
Stevenson was a national favorite of, like, educated liberals, who by this point had absolutely seized the Democratic nomination process (JFK, who was good at pandering to these people, would appoint Stevenson as UN Ambassador, which was an excellent pander), but he was indeed not necessarily very popular with the voters. The 1950s were arguably the first decade in which "educated liberals" were a real faction of the electorate -- complaints about the liberal culture of universities go much earlier (back to the 1910s), but my understanding is that there wasn't actually a bloc of voters like this until after the Second World War.
Funny to consider how the DNC making nomination decisions would go nowadays. We would absolutely have gotten Mayor Pete as the candidate in 2020.
Yeah I think this kind of boils down to a fact that Stevenson was quite frankly a lousy candidate. People don't think of him as such nowadays and he's generally well regarded and respected even by Republicans because there's not really any real reason to hate him. He didn't do anything particularly controversial as governor, he had no scandals, and he was on the "right side of history" on the noted issues of the time including civil rights (although having a segregationist as his running mate in 1952 is a blemish but not one particularly known), but that doesn't make him a good candidate. Walter Mondale and even Michael Dukakis are starting to be seen similar to him too.
The fact that the Democrats nominated a weak candidate both times is no surprise, once Ike announced as a Republican 1952 was basically locked up (and probably would've been anyway with Truman's unpopularity), and in 1956 Ike was so popular and unbeatable no one else wanted to run so he just got stuck running again. But it's still a fact. Stevenson was a good guy, but a very weak candidate.
