Mayyyyybe the final level was not as important for a long time, when they still followed the original story of rescuing hostages and getting photographic evidence of the bio weapons on the planet. With the switch to the simpler fusion core collection quest, the Spaceport where the giant explosion is triggered got a lot more important and they planned to do a special graphic set for it, but ran out of time. Just theorizing here, of course
I'm happily plodding away at the map editor.