Charged TM keeps giving me the same two moves

I have a level 35 Gyarados with perfect IVs but it had Crunch as the charge move so I wanted to change it to Hydro Pump. I had 6 charge TM lying around so I decided to roll the dice and use them. I ended up using all six charged TM and move kept switching back and forth between Crunch and Outrage. I never got Hydro Pump at all. My question is...

Was this just bad luck? Or do some pokemon have certain Charge moves blocked? I'm betting it was just bad luck but I figured I would ask because six TMs is a lot to have wasted.

You just had bad luck. The only blocked moves are the legacy moves, but I suspect you already knew that. Different moves are all acquired with the same even probability. Since there are three possible moves there is a 50% chance for each TM use to switch to the preferred charged move. But since how math works that means that only at a limit of infinite TMs you would certainly get it. The probability of not managing to get the move as you described is (1/2)^6 which equals approximately to 1.56%.