Until Harold Shipman, John Thompson was the perpetrator of the worst mass murder in British history, which took place as recently as 1980 - so why isn't he notorious? Part of the answer, the Independent's Simon Usborne implies, is that the victims of the arson attack for which he was responsible were portrayed in the media as low-lifes whose deaths didn't matter. This attitude extended to the authorities - as both of the clubs in which people died were illegal, it was not felt necessary to hold a public inquiry. As a result, the horrific episode has disappeared from the collective memory. Credit to Usborne for doing some digging in the hope of giving the dead the dignity they deserve.
(Thanks to Neil for the link.)