when the engine fails in a single you have to land, sometimes messy, very occassionally fatal.
when the engine fails in a twin, sometimes it is non-eventful, sometimes it is an accident and sometimes fatal.
BUT sometimes in a twin you also have a fatal accident that you would not have had in a single, from other causes. the critical components have to work harder to pay for for the occasional imunity. Less engine failure accidents paid for by more gearbox/tailrotor/freewheel units/fuel systems/ performance cost/ tail boom failures/ 'pilot error' (shut down wrong engine, confusion etc) etc
All things will happen in aviation to some extent.
PROPORTIONATE REGULATION does not call for twins.
the sums don't add up, gazelles just did not have an engine failure rate that justified twins FULL STOP