The reason why: Cheerilee and Scootaloo are sisters. So there parents must have been an earth pony and Pegasus. Since they are a family, they must of had to grow up on land, showing how Scootaloo doesn't know how to fly very well. That's why she looks up to Rainbow Dash as a mentor because she's a great flyer!:

This was from when scootaloo was an earth pony. I still like to think that they are sisters and there mom was an earth pony and dad was a pegasus.

Rainbow dash: Tweet tweet tweet   Fluttershy: Tweets back at Rainbowdash   Twilight: *Comes to the pond* Quack quack quack   Celestia: *Flys in from out of nowhere* HoNk

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