@digital_carver wait, what? Taft is pretty well-known for not really wanting to be president, but taking the job until he could be appointed Chief Justice. Plus he had an important role in the story of Teddy Roosevelt, who's even better known.
And then there's the reason all the schoolkids remember him: being so fat as to get stuck in the White House bathtub.
@khird I think it's sort of what @namark said, but indirectly and unintentionally.
Taft was famous enough that the podcasters, comedians, etc. remembered his name (the others too forgettable even for them), but didn't stand out as significant compared to the others they had in mind, the Lincolns and Kennedys.
He just barely passes the significance threshold to be on the fame ladder, but only to get on the last rung of it.