Nah, I just think there weren't any Pegasi or Unicorns to begin with. Head canon time!
Cut back to the time frame of the Hearth's Warming Eve story. Some of the Earth ponies decided to stay and continue farming the land that had been in their families for generations instead of joining the others in their mass exodus. In spite of their efforts they were still unable to get anything to grow, and now they even lacked their scapegoats (Scape-ponies?) to blame it on. Instead of just waiting for the end the remaining Earth ponies pooled the last of their resources and had a festival so they could go out with a smile. The resulting happiness powered the Crystal Heart and BAM! They once again had farmable land. The land now being maintained by the power of crystals, the Earth ponies intrinsic connection with said land caused their metamorphosis into Crystal ponies.
And that's how Pinkie Pie got her cutie mark!
Yeah, I put way too much thought into these things.