It also has to do with departure times at FRA. The flight to SIN needs to be planned at a time that people can actually make the connection from their feeder flight. The outbound flight from FRA already leaves at 21:30 or so and later slots for big aircraft are already taken. Plus that late departures of intercontinental flights at FRA always get a stressed out cabin crew and pilot on the intercom talking about how 'we cannot miss the slot time' and 'flight restrictions at night at FRA'. So even if you depart 2 hours later, you'd still be having over 5 hours left at SIN for a stopover. The other way around, like with making the flight much earlier and having a turnaround much earlier would raise another issue: the aircraft would probably land late in the evening at FRA, making it impossible for LH to sell further connections from that flight: no one voluntarily buys a flight with a long waiting time and an overnight connection at the airport.

By the way, the same happens at many more airports with many more airlines. I don't know about many airlines who are able to operate a schedule where all their long haul metal is constantly in the air, actually.

The good thing is that they now have good time for the turnaround. Having good time for a turnaround is something DY still hasn't understood.