Just a bit of speculation and headcanon. Some may have already thought of this, but I wanted to point it out just in case; Discord was reformed because Celestia wanted to use his magic for something. This was only a few episodes before, whaddya know, Twilight gets wings.
For a moment, let's forget Cadance exists. In my headcanon, I think Discord was orginally good but turned evil, much like Luna. Perhaps he worked for the royals, or was the biggest ruler of Equestria. His magic is powerful and unlike others, and we know he can change the species of a pony as he made the enitre mane six earth ponies in his first episode.
Twilight, orginally a unicorn, was made an alicorn because she earned it by mastering magic. I saw once on My Little Brony that someone had noticed a connection between all of the princesses; their designs changed over time. Luna's is very different between her apperance in season 1 and in season 2, and Cadance was pink in flashbacks but later on her wings were purple at the tips. That person was thinking about how Celestia's toy is pink, and about how it seemed that alicorns were made, not born. So Celestia may have been a pink unicorn (Which, if you think about, is close to the description of G1 Twilight), then earned her princesshood. My whole point of this paragraph was to point out that there may be more to alicorn than just wings, maybe they are all naturally unicorns with spells cast on them that changes over time.
My conclusion with all this thought is that Discord, before turned evil, may have been the one in charge of picking the rulers of Equestria and only his magic could make a pony an alicorn. Thus, he made Celestia and Luna alicorns before he turned evil. In order to give Twilight wings, Discord needed to be reformed and willing to use his one-of-a-kind magic.
Now, for all of this to make more sense, I should adress Cadance and why I said to forget about her in the beginning :P. She may be the reason this arguement falls apart. She is young, only as old as Shining Armor, which means she would have been born long after Discord was banished. I have two theories; 1.) Alicorns, living longer than normal ponies, age much slower as well. She could have been born before Discord was turned to stone, and aged so slowly that by the time a thousand years rolled around she was Shining Armor's age (thinking in maturity, not actual age). Along with this, she may have been too young to earn it and Discord turned her into an alicorn in his choatic rebellion. 2.) Alicorns are acutally born and she is related to the royal sister enough to have inherited this rare gene.
So, does anyone have any thoughts on this? Anything to add or any holes to poke?