Exactly as V Jet said,
It's due to the noise curfew in Sydney.
This affects all Qantas' Europe-Australia flights, because due to the distances involved, and the time-zone differences there are very few departure/arrival time combinations which are commercially viable. A later arrival into SYD at say 8am would mean a 3am departure from FRA!!!
Heathrow services are even more critical because LHR also has night flying restrictions, and Qantas have carefully scheduled their flights to fit in with the restrictions at both SYD and LHR.
Ausralia-Europe flight timings are less critical, and Qantas has obviously made the decision that these times are commercially viable. And as TWA@fra said, the arrival/departure times at SIN/BKK on both legs require some consideration here.
As for the idea that business travellers are going to do a full days work in Germany, fly for 21 hours and cross 8 time zones, arrive at 5am and then do a full days work in Australia is a little unrealistic. I doubt many companies would expect employees to work effectively make sound business decisions just a couple of hours after stepping off a flight to Australia!
Don't think anyone on airliner.net is qualified to pick faults with Qantas' Kangaroo Route scheduling, Qantas were doing this before 99% of people here were born so I'm prepared to trust their judgement