I'm going to answer this for why Duraludon doesn't run Light Screen and Reflect in NU, as Duraludon is a PUBL Pokemon who has viability in NU. There are 3 main reasons for this.
1. Duraludon's role is a Wallbreaker
Duraludon is used as a Wallbreaker with its excellent coverage moves. For those who don't play NU, Duraludon's coverage is actually pretty good for NU's standards. Draco Meteor just being a strong move in general, Flash Cannon for Fairy-types, Thunderbolt for Water-types like Vaporeon and Mantine, and Body Press for Steel-types like Copperajah and Stakataka. Duraludon is used as an Expert Belt wallbreaker or a Draco Meteor + Eject Pack wallbreaker. Duraludon running Light Screen and Reflect means it would have to give up two of its coverage moves, which isn't really desirable for it.
2. Before the Light Clay Ban, there was a better dual screener
Another reason Duraludon didn't use Light Screen and Reflect pre Light Clay ban was because it would be outclassed by a Screens Pokemon named Xatu.
Xatu was the premier Screens setter for Screens HO when Light Clay was allowed in NU. The reason was for it due to having Magic Bounce to prevent hazards from going on your side and Defoggers from getting rid of your screens, as well as having the most important reason, Teleport. Teleport would generate momentum and would allow you to bring in a teammate safely. Technically, Duraludon could've used Steel Beam with Light Screen and Reflect, but it wouldn't provide the same utility Xatu had.
3. Light Clay is banned
UU had banned Light Clay a while ago, and that ban effect means Light Clay isn't useable in NU. This made Screens HO not as good when Light Clay was banned. Even if you wanted to make Screens HO in NU without Light Clay, Xatu would still be used for Light Screen and Reflect. Heck, you could even make Grimmsnarl a Screens setter since it has Prankster and Taunt. A Grimmsnarl set with Eject Pack, Light Screen, Reflect, and Superpower could be a cool tech to help generate momentum.
TL;DR Duraludon doesn't run Light Screen and Reflect because it serves as a wallbreaker utilizing its coverage, and Xatu outclassed it as a Screens setter, before and even after the Light Clay ban.
Do note this just covered NU, but for other formats Duraludon is viable (which would be Monotype and National Dex Monotype on Dragon-type teams), Duraludon is used as a Choice Scarf Pokemon to be able to revenge kill Pokemon.
Hope this helps! :)