Why can only earth ponies grow food?

[Marcusaralius76 150]

Because in order to fly, the peggasi would need incredibly light frames, and the unicorns don't have much muscle because they do their work with magic.

 

It's not the Earth Ponies' fault that they're the biggest and the strongest.

 

 

Also, remember that Ponyville is predominantly earth ponies. It's possible that other towns with a more diverse population would include the other groups in farming.