Diacetylmorphine is just a shortened version of (5α,6α)-7,8-didehydro-4,5-epoxy-17-methylmorphinan-3,6-diol diacetate
People call it heroin because it's the easiest thing to call it.
Commonly used drug names don't always make the sense we'd like them to. Heroin stuck. Heroin is fine, everyone knows what's meant when someone says it.
And diacetylmorphine doesn't have the same familiarity?
Could be worse, the media in Australia constantly labels methamphetamine as "ice" - the term is now so widespread people outside of the drug scene believe meth and ice to be two different things.