DC's Superman is well-known throughout comic circles as the Last Son of Krypton, the sole survivor of his homeworld and his race. While other Kryptonians have been introduced throughout the DC continuity (especially Superman's cousin Supergirl), Superman held the title as the last survivor for many years. However, Krypton is often depicted as a highly-advanced society in almost every continuity - and in almost every continuity, the Kryptonians had mastered space travel. With that in mind, why was Superman the only survivor?
In the modern age of comics, a terrible war had engulfed Krypton and reshaped the planetary civilization into a cold and isolated society. The scientist Jor-El, Superman's father, eventually discovered dangerous radiation building in the planet's core, and begged the Kryptonian council to leave the planet - but the staunchly isolationist council had forbidden space travel, and didn't believe Jor-El in any case. But even when Krypton's leaders realized Jor-El was right, they still didn't evacuate. Perhaps the rulers would rather die than break tradition...or perhaps there was another obstacle they believed could not be overcome.
A Kessler Syndrome, theorized by NASA scientist Donald Kessler, is a worst-case scenario that effectively dooms a population of a planet to remain on said planet for decades, if not centuries. Debris from satellites, rocket fairings and abandoned tools and equipment is quite difficult to track (and when one considered just how small some debris can be, almost impossible), and despite the vastness of space around a planet, the likelihood of abandoned satellites colliding increases with every launch. A society as advanced as that of Krypton would surely have hundreds if not thousands of orbiting satellites...and as per the Kessler Syndrome theory, one collision could lead to another until navigating a spacecraft beyond orbit would be akin to a ship passing a dense minefield.
This is why Superman's rocket left the planet - it was the only one that could. As a craft that could only hold one occupant, it would be small enough to navigate the Kessler field. This could also be another reason why Jor-El was ridiculed by the scientific community of Krypton; the council would have thought traveling through a thick debris field simply impossible with massive evacuation ships. This is also, tragically, why Superman's parents could not accompany their child to Earth.
For all the pseudoscience that comic books present their readers, every now and then a plot development is based on an actual scientific theory. A potential Kessler Syndrome is a very real threat - one that Earth could face quite soon if the accumulation of space junk in orbit isn't halted. Superman may have been Krypton's only survivor because the ruling body of the planet failed to listen to reason, but our own planet still has a fighting chance.