There were actually two serious accidents, one resulted in a death. That plus the unstable area it was built on basically caused its removal. It wasnt worn down or in bad shape, it was just a death trap in the parks view.
One accident involved the drive system failing on a train going up the lift, no anti rollbacks, it then hit a train in the station. The other involved a train failing to stop as it entered the station, hitting a train there. I forget which one caused the death.
This is from Wiki but it explains what happened...
http://en.m.wikipedia.org/wiki/Whizzer_(roller_coaster)