This is because Magikarp's ORAS moveset page (the first one you see when you view Magikarp on the DB) only shows those moves that it can legally get in ORAS. This means that all moves it could have learned, from FireRed/LeafGreen and R/S/E to ORAS will be included.
But the Dragon Rage event (the University Magikarp) happened in the Red/Blue/Green versions of the game. Any Pokemon from these games is not transferable to ORAS, hence making it an illegal move on Magikarp. So that's out.
However, there was also a more recent Black/White event in which Hydro Pump Magikarp was distributed. Now as for why that's not there I'm not sure.
Hope this helped in some way!