Work Up runs counter intuitive to Superpower and Draco Meteor. 2 of his better moves, and basically staple for being a Mixed threat. His movepool is very shallow: what moves would he replace those 2 with? If you replace Draco Meteor with Dragon Pulse, you are effectively running a weaker Latios/Calm Mind sweeper if you want to hit hard on the special side. Latios can try to sweep with Dragon Pulse, backed with CM, in part because he's also a faster Pokemon and because CM will buff his Sp Def. There is NO point running Work Up Hydregion with Dragon Pulse, and running Draco Meteor again is counter intuitive with the Special Drop.
Superpower has NO comparable move. I don't even think he learns Brick Break. He uses Superpower because it's very strong, and because it's Fighting which gets around Tyranitar Blissey etc. That's how he overpowers through some of these Pokemon, because his attack stat is not overwhelming enough for him to be landing neutral coverage blows and win. You can't run Facade or Earthquake or xyz other move and be nearly as effective. This is especially true because Hydregion cannot afford to split his EV's along Attack and SA. He generally needs as much Speed as he can get, or else be otherwise entirely useless (and even then he's fairly easy to outspeed). So you're left with 252 EV's for both stats, can't cover them both, and putting them all into SA makes the most sense for Draco Meteor to hit hard/he's inherently more threatening on the special side to begin with.
Hydreigon can't learn SD or Nasty Plot. The point was that there are mixed sweepers with access to those moves, who are better off using those moves to begin with because they boost quicker. If Infernape wants, he can run Nasty Plot with a back up coverage of Close Combat. No Work Up needed, etc. It's not a great move because those other boosters exist, and because choice items can give you the instant power on the "usual" side (Choice Specs on Hydreigon for example) and you can still run the other side move like Superpower which will hit exactly as hard as a Work Up set before the setup. After the setup, being +1 isn't that helpful because your main purpose in the first place was to surprise them with the move, or to hammer away at them with your main attack like Draco Meteor, and then have the opportunity to respond.
The very fact that Hydreigon HAS Superpower, is more important than his ability to boost it. That's the point of some of these Mixed Attackers: Salamence doesn't need Work Up for Fire Blast to roast Steels. Infernape doesn't need it for Grass Knot to **** a bulky Water. Tyranitar doesn't need it for Ice Beam to randomly **** on Gliscor trying to wall it. Etc. The mixed moveset being present IS the power, IS the draw to using it and succeeding with it.