GamePress
Why do legendaries cost dust to power up
1) Dust is gained by catching pokemon, you don't enounter wild legendaries.
2) Legendaries can't be left in Gyms so their power level is moot for defending gyms
It would make more sense for Legendaries considering thier unique method of capture (raid only) be powered up through raid only methods (ie candy only because you get candy on catches and on rare candy)