It was a huge balls up from the staff at the time. But like with many things, the safety procedures weren't entirely up to snuff and the way it which the park was being run arguably led to the mistake being made.
It changed the course of the park and Merlin a LOT. Sadly it caused a major hit for the industry that it has struggled to recover from (mostly as, in my opinion, Merlin went into damage control mode rather than focusing on getting the parks to a level of quality again). Combine that with the general state of the world in those 10 years and its no wonder we're sat here begging for Universal.