Crisis on Infinite Earths, which I talk about a lot, retconned Superman into being the sole survivor of Krypton’s destruction. Erased from continuity were Superboy (the teenage, Smallville-inhabiting, Legion of Superheroes-inspiring Clark Kent) and Supergirl (Superman’s hot cousin). Clark Kent didn’t begin donning a cape until he was an adult andnobody else survived the explosion of his home planet.
The problem with the old ‘New DC’ that resulted from the first Crisis was that the supporting cast of the Superman books was kind of a drag. The Guardian and Gangbuster barely made an impact and before long Supergirl and Superboy were re-introduced to continuity, though Supergirl was a shapeshifting alien from a parallel universe and Superboy was a clone made from Kal-El’s DNA, which meant both characters were still playing by the rules established in post-Crisis continuity.