Because Cynthia is apparently some god mode trainer that can't be defeated at all. No matter how good the trainer, she still wins like it was just a cake walk.
Seriously, I'm one of the few that hates Cynthia. The writers turned her into some unbeatable, smug little ***** that apparently knows everything about the Pokemon world. Even in an unofficial battle, one that would not blemish her rank as Champion in no way, she was made out to be the perfect trainer.
I know that she is a Champion, but she just comes off as smug to me. Even more so than Paul. She walks around and pretends to be humble, but she just comes off as a know-it-all.
I just don't like that just because she is a Champion, that she isn't allowed to lose. But at the same time, she is allowed to utterly destroy anyone who dares to try to take that away from her.
So in a sense I'm rooting for Ash to beat her. Not in an official Elite 4 battle, but an unofficial one like Paul. I say Ash will get as far as Lucien and lose to him. But she will be so impressed by his skills, she challenges him to a battle. And in the end, Ash comes out on top, but promises an official battle in the future.