They revealed other things. Besides, this wasn't a mystery from the first episode (the Skye mystery then was whether she was a traitor). It was arguably a mystery from Girl in the Flower Dress (she revealed she was interested in SHIELD because she wanted to find her parents), then there was the hint about the SHIELD agent being found dead (can't remember which episode). Then it didn't really kick into high gear until Seeds (episode 12) when they reveal she's an 0-8-4.
They've revealed more since then. There is merit to proper pacing. When the show has several mysteries all at once, they're not all going to get resolved all at once. What's up with Ward was mostly solved. Who the Clairvoyant was has been solved. How Coulson came back from the dead was solved but a new mystery was added to that.
What's up with May and what's up with Skye are the two big ones. That doesn't sound unreasonable to me. Shows with more than season don't have to resolve everything by the end of the first season.