The short answer is "southern historians." The lost cause narrative that emerged after Reconstruction lionized Lee's generalship in service of what Grant described as "one of the worst (causes) for which man has ever fought" and portrayed northern generalship as supremely bloodthirsty in putting down that cause. These historians left no stone unturned in smearing (sometimes appropriately) Grant's reputation, whether his alcoholism or the machine politics and corruption of his administration. Imagine if the modern day hit pieces against Obama and the Clintons were the predominant record of their accomplishments, that's pretty much how Grant was painted for 50 years. It's the most notable exception in the modern era to "the victors write the history."

A shift from great-man approaches to a more trends-and-forces kind of analysis resolves a very different picture of his administration. He was confronting a conquered rebellion and attempting to re-integrate it into the fabric of the Union, while at the same time his Republican party was more fractured, arguably, than either major party today. He had enemies in the more liberal wing of the Republican party for being insufficiently forceful in the dominance of the South. Viewing Grant's administration in light of the social and political realities in which it operated paints a strikingly different picture of his competence.

As both methods of historical inquiry have changed, and as historians (and their audience) who started with an a priori premise that he was a monster have died off, the light has become more favorable.

I'm not super familiar with that ranking, but my best guess would be it having a lot to do with the evolving historiography on Reconstruction, which he was the only real Presidential supporter of.

That it, until quite recently histories Reconstruction (and to a lesser extent the Civil War in general) were dominated by the Dunning School viewpoint, which grew out of the Jim Crow South and portrayed Northern Republicans as rapaciously looting the south, Black office-holders as almost uniformly corrupt and incompetent, etc.

The increasingly universal viewpoint these days is to view reconstruction as, to just quote the title of one of the more influential books on the subject, "America's Unfinished Revolution." That is, as a noble attempt to secure civil and political freedom for the Black population ultimately undone by white Southern terrorism and Northern apathy.

In the first paradigm, Grant is a corrupt drunkard whose entire presidency was mired in corruption. In the second, he was the last President to actually make an effort at protecting a vast swathe of the population for nearly a century.