No one's saying Stone Edge is a bad option. It's a good one. The thing is, Lucario can't run all the moves it would like to due to four moveslot syndrome, so no matter what you choose re: Crunch vs Stone Edge, it's going to run into some problem or the other.But what about the coverage gained on gyarados and zapados gained by stone edge?
Whether Lucario should use Crunch or Stone Edge depends on the rest of your team, honestly. For example, if they can walk all over Rotom-A or has a Gyarados or Zapdos problem, then Stone Edge will probably be the better choice. If they're fine with Gyarados or Zapdos, but have ghost issues, then Crunch is the way to go.