It's also because Ash going with a speedy electric type was something Cress was prepared for, as a lot of beginning trainers have probably tried the same thing. And, no offense or anything, but Pikachu being an electric type doesn't matter much if he doesn't even land a single hit. But yeah, Cress's speedy attacks did impress.
Honestly Pansage may have actually pulled out the weakest gym battle out of the three.
Loved the episode too, by the way. I'm glad Ash wasn't completely reset as you can see by his strategizing here that he does know what he's doing. I am a bit sad at Pikachu's one-sided loss, and the fact that this was the only real interesting gym battle so far, but I can't complain.