Keeping in mind the unexpected commercial success of G4, showing the majority of Earth Ponies in the main cast being involved in food prep was likely the easiest category to write for at the beginning, connection to nature equals food. Don't think the writers really accounted for any racial discrimination topics, because if the protagonist is a unicorn then why do the earth ponies need special powers?
The reason for the conflict amongst the ponies in G5, besides an obvious need for the new gen's convenient premise, can be seen as the eventual outcome from lack of a common enemy.
It's always the same in any story, different personalities and cultures are only brought together when they face a mutual threat. Take that threat away, and tensions will gradually rise from small differences regardless. The reason they'd still want to fight despite now being on equal ground is simply because they want to, conflict is bred by a desire for competition, and that need will only grow more from the boredom of peace.
Tell you what would really make them mad, trying to understand how they can physically operate electronic devices without fingers. I try not to think too hard about it as I'm typing this, being an earth pony and all. Haha