Why? Because the writers of the character are horrible at their job and it showed.
The character is an unlikable douche who's power comes not actually from his backstory, but rather, from terrible writing of the past all gathered together and left in a pile for the readers to deal with. Honestly, if not for the writers making him the equivalent power to the epitome of all supermen across multiverses, he'd not even warrant consideration. And that is a mark of a bad villain, not a good one, if their powers is all you can recall about them.
The character makes me think of bad fanfiction, where someone mary-stu'd all over established cannon and ass-pulled about scope of the power as a poor attempt to make the character threatening, while sabotaging their own efforts by having the character be the most whiny, annoying brat readers are forced to deal with. Hell, they even "nu uh'd" the standard superman weaknesses with bullshit, so that kyptonyte barely worked and red sun radiation merely reduced him to the power of the standard superman instead of depowering him completely. Hell, spoilers if I remember right, they couldn't even kill him, they had to literally cage him within a red sun with a swath of green lanturns constantly watching just to keep him pinned down.
Good villains can express aspects of humanity gone astray in a way that makes the character threatening beyond their power. The joker, not counting some weird eternal life stuff going on recently, was always just a madman. A single man who's threat was minimal in skills and ability but who's threat was super villain territory because of madness and unpredictability. Lex Luther is a single man who's intellect is indeed powerful, but it his ambition and drive that makes him a good villain (when done right anyways), as it is ambition that makes him threatening.
Superboy is what happens if a toddler was given the power of a god. The result is as villainous as a natural disaster. Sure there is a lot of destruction, but it is so pointless in the end, and not impressive outside of the scope.
Now the character had potential. It had a lot of potential as a concept. A being who is a god, who's power is so vast and dwarfs even the main universe's superman grieves for the loss of his universe and wants to right the world regardless the cost. That is a decent motivation there. The power of loss mixed with the the faint hope of fixing what was wrong all mixed together in the mind of a teenager who's pain fueled the direction he started down, and who's pride made him see it as the only way once it started to cost his soul and his morality as he went. Even more, the character would have sympathy and if the writers presented it well, could actually argue the point with some degree of reason, making the efforts that much more fitting. Superboy was a hero, if the situation was presented as, say, the universe could be fixed so he had his lost ones back at great risk to other universes, then it pits the heroic idea in his own head of fighting impossible odds against the risk of great harm the others see much more clearly. Many good villains see themselves as the hero after all, and of those that do, the best ones actually have a point.
Sadly, none of that gets through well with the character at all. The grief is displayed as petulant whining. The determination conflicting with moral uncertainty about what he was doing barely mentioned. The situation very black and white, and the character actively knows it means the end of everything, even from the start if I remember right.
The character is just bad, and the only villain I see when I think of him is the fanfic writers
