Well... if you have a tank in which you can replicate the size of their territory in a river then good luck to you mate!
Fish in a river arent kept in one place in a glass box, its constantly changing and the discus guard their eggs, they chase away would be scavangers which will just move on to find something else to eat. In a tank the discus cant make them go away, they are constantly under threat and trying to guard their eggs which 99.9% of the time will be eaten if there is other fish in the tank. Either by the other fish or by the parents when they get to a point they are so stressed that they give up trying to rear the fry and just eat them.
Perfectly clean water: well with the river water rushing past and not stagnating, its constantly at the right requirements for them and constantly 'clean' in terms of pathogens.
Minimal Tank Decor: Easier to keep the tank clean! Even the slightest nitrate from fish waste in gravel might kill the babies not to mention its impossible to clean when babies are in the tank! The babies are less tolerant than the adults of impurities in the water.
It also means you can control to an extent where the discus lay their eggs (on the cone instead of plants) which if needing to move them etc is easier done...
Baby fish cant swim in constantly high flow, they end up getting very weak and dying, goes for most fish but thats all there is to that one really...
Dont know what you mean about constant temp changes? You generally want to keep discus at about 27-28 deg C and raise it up to 30 deg C when they start spawning. At a guess its to do with time of year that they breed in the wild? Possibly after it has rained and dropped the temperature and it starts rising again? Or maybe the move to shallower water to spawn in which case the water would be warmer than deeper water. Something like that
It all boils down to the fact that it is incredibly difficult and very impractical to breed discus in a biotope tank that isnt HUGE. Like 10 x 10 x 4 kind of base size for a start... but even then other fish would pick off the babies. Most people breedingdiscus want all the babies to survive, not play pot luck and just accept 'as many surviving as possible'.
