It failed because it was an overlay instead of a new design
Of course, not all overlays are failures, but the magnitude of this overlay was so much that damage was done
1. There was major structural damage done to the tracks. The tracks were designed to hold what was basically a slower version of our own TTA. Rocket Rods, however, travelled much faster and changed speeds often.
If they would have rebuilt a lot of the track, they could have banked turns and the cars could have travelled much faster.
2. There were no major additions to the ride. They made minor changes to the tunnels, adding some lights and strobes, but nothing major. The scenery changes were all very very cheap.
3. The lines for the ride were rediculous. The ride's capacity was even less than Test Track, if that gives you any picture. The vehicles only held a whopping 4 people per car. Of course, before the single rider line, many seats would end up being empty.
Even with the negatives, the ride was still a big hit. The section over the Autopia was very thrilling and the cars actually went at a high speed. The tragedy here is that the cars went pretty fast in the Autopia section, but slowed down a lot to go through an S-curve because they didn't build more supports to straighten out the track and keep the high speed rolling.