I think the route HAM-JFK doesn´t have enough traffic expecially to fill
a widebody aircraft.

Remember Lufthansa commencing a JFK service with the routing TXL-HAM-JFK with a twin-jet aircraft A310-300-ER in the 90s. Lasted one or two years and was subsequently cancelled.

Argument that there was although the economy class has been booked very good there was not enough premium fare traffic (Business/First) and so they decided not to offer it anymore.

Remember it very good because that was my first domestic flight with an intercontinental aircraft which was much more spacier than the B737/B727 which was used at that time between German airports.

So I think the argument may be the same in nowadays environment due to the fact that there is only a small number of companies in Hamburg with much intercontinental air traffic.