Can I ask why trips like SIN and HKG pay such huge box payments?
Presumably because they are so long and arduous that it is a form of compensation?
In the later part of the 80's the 747-200 started to fly routes that were longer ranges than the current scheduling agreement, ie it could fly beyond the longest duty day currently flown which was the LAX flown under the "West Coast Agreement". The new routes were HKG/BKK/NRT/JNB and a deal was struck with the union for sector payments and rest facilities hence the "Wendy House" in the back of 747-200 World Traveller cabin to give cabin crew bunk rest.
Along comes the 747-400 in 1989 that scoops up the HKG/BKK/NRT/JNB and now could also do SIN/EZE/KUL, it had dedicated crew bunks and the sector payments were scaled up, formalised and became set in stone.
In reality and you won't find many that will disagree (if they do, scheduling will happily substitute them from a long range trip and find them a MIA), with extra crew and dedicated bunk rest plus a significant sector payment, the "Long Range" sectors became far more popular than the MIA/SEA/YVR/BOM whatever.
The short answer is that the majority of "Long Range" sectors are relatively straightforward, not particularly arduous and much sought after compared to the daylight MRU/MAA that do not attract a comparable premium payment and are considerably more demanding.
Of course that is a personal opinion..