Uh... nobody's perfect, I guess was the prevailing thought? Fwiw, they did address that topic:
Considering drops for Andrew Jackson (ninth in 2015 to 21st now) and Woodrow Wilson (10th to 15th), Rottinghaus and Vaughn noted the impact of campaigns for racial justice.
“Their reputations have consistently suffered in recent years as modern politics lead scholars to assess their early 19th and 20th century presidencies ever more harshly, especially their unacceptable treatment of marginalised people,” the authors wrote.
Jackson owned enslaved people and presided over the genocidal displacement of Native Americans. Wilson oversaw victory in the first world war and helped set up the League of Nations, but was an avowed racist who segregated the federal workforce.
(emphasis added)
So he did drop from 10 to 15 for this reason, but I guess winning WWI still kept him high.
I dunno, perhaps they should change the ranking to throw in some additional items like foot fungus, head lice, 2nd-hand dog vomit and such... and then Trump would remain in last place!? :-P