Everypony knew that when it came to ponies, a horn was an absolutely essential requirement for casting spells. Other creatures could channel raw mana without one -- dragons, for example -- but there’d never been a pegasus sorcerer, or an earth pony wizard. Magic was pervasive, and practically every creature had some connection to that source of power, but for whatever reason, those two pony species didn't get the direct hotline to mana that unicorns did.