Page "Superman: The Animated Series" Paragraph 4

The producers of the show decided to make Superman more believable by limiting his powers so that according to them he could probably be killed.

Another noteworthy change was to Superman's origins and that the evil computer Brainiac is not only from Krypton, but is portrayed as responsible for preventing the knowledge of Krypton's imminent destruction from reaching its people so as to save himself rather than be committed in the presumably futile task of saving the population of the planet.

In addition, the ship that carries the infant Kal-El to Earth is designed to have a pilot, and the autopilot used instead was programmed to land smoothly upon reaching its destination.

This was done so that the ship is in perfect working condition during Superman's adulthood and could be used as his mode of long range transportation in space.

In addition, access to Kryptonian technology and artifacts is initially severely restricted such as the ship containing a phantom zone projector and Braniac's technology, although Superman later finds a devastated colony in Krypton's solar system with partially salvagable technology in addition to Kara In-Ze in her functioning cryostasis capsule.

Also according to producer Bruce Timm, they wanted to use Supergirl early in the series but the producers put it off until late in the second season with the only exception was that she could not come from Krypton.