Rogue One worked because it provided plausible explanations for points about the original film that didn't seem to make any sense. It seemed impossible that the Death Star could be destroyed so easily or that the rebels could find that flaw so quickly. Now we see why. Solo fell flat because nothing about Solo's past, as depicted in the film, significantly changed how we see his character.
I suppose Joker worked for some people because every version of the Joker is different so you could look at Joaquin Phoenix's Joker as a stand-alone character driven to insanity through various things in his life. It surely didn't work for anyone who expected an origin story for a version of the character that would eventually turn out anything like Heath Ledger's Joker. It's just not possible to draw a line from one to the other.