Structural damage to the exterior track caused by the Rocket Rods is part of the reason it hasn't come back.
Based on things Tony Baxter has said in various interviews, Imagineering has tried to figure out ways to bring it back over the years with no success. Original building codes and regulations would not be grandfathered into today's standards. He said today's codes would require evacuation stairs every so many feet along the exterior track, and the vehicles would need enclosures to prevent people from sticking their arms out during the interior portions and widening the interior tunnels would require major renovations to the buildings.