It's exactly about safety!
The complexity of the aircraft AND the task may require a good level of twin experience. You can not get twin engine operational exposure and experience by flying in a single. What about AOC at night, in IFR, offshore, police, etc?
The owner/operator will want proven experience and competency before letting someone loose in a $5-20M helicopter.
Most of the onshore twin positions require Captains so there is little opportunity to train/coach a low/no time twin pilot.