It's because they're evil!
The idiots in the media have demonised the very word "derivative" without understanding for a moment that it is not derivatives that are "evil", but rather abuse of leverage...which abuse (on a serious basis) began in the late 1980s and continues to this day. By objective measure, this abuse has almost surely gotten worse over time, as banks et al. attempt feverishly to recoup their losses from 2000-2002 and 2007-2009.
Just call me Sgt. Schadenfreude, eh?
;^)