When Equestria was first founded, it was done so as a coalition—a collaboration of mutual benefit between the unicorns, pegasi, and the earth ponies. After the nasty incident with the windigoes, not a one of the three races was willing to take any chances; therefore, all were happy to comply with the notion of all three races having equal power in the new nation. After all, what sort of a fair government doesn't represent its ponies?

For a time, this government seemed to work well; Chancellor Puddinghead, Commander Hurricane, and Princess Platinum were able to work together and find common ground, to make sure the fledgling nation was running in the best interests of all its citizens. Months passed, and the ponies of the capital city of Canterlot led good, resplendent lives. Meanwhile, farmers and ponies of the like spread out all over the landscape, dotting dozens of tiny cities onto the map.

And yet... they knew this could not last. It had always been an undercurrent thrumming just below the surface, sure, but then Commander Hurricane fell ill. Deathly ill. It turned out that the poor mare would have just days to live, if that. So they all convened around her hospital bed and devised a plan.

It would be unfeasible to replace any member of the trio, for any new member would not have had the same experience with the windigoes as the original three. As such, they might be more inclined to power grabs or something of the like.[1] No, the three of them decided that power could not fall into the hooves of the common ponies; they wouldn't quite know what to do with it.

But what else could they do? Commander Hurricane would die in the next few days, and the other two knew they didn't have much time left. And yet they couldn't agree on anything; all they knew for sure was that it had to be somepony impartial—somepony with no stake in the matter... or, even better, somepony with a vested interest in all three groups. But alas, no such pony seemed to exist.

It was then that their assistants were sent to scour the land. Clover the Clever went up north to the Crystal Empire, Private Pansy talked to the ponies who lived in the outskirts of the Everfree Forest, and Smart Cookie talked with local farming communities. 

Sometime, a dozen or so days later, they found her. Pansy's and Smart Cookie's paths converged at just the right moment—each trotting in either side of a massive cavern somewhere deep in the Everfree Forest. Their eyes locked as they met the centre, and then they looked down to see the creature in the middle... and she was beautiful, unlike anypony they'd ever seen.

As Pansy and Smart Cookie stared in awe, the white bundle of fur and feathers pushed herself up from the ground and brushed herself off with a hoof, pausing only to run the hoof through her bubblegum-pink mane. She introduced herself as Celestia, and claimed to be able to raise the sun in the day and lower it in the night. What Smart Cookie noticed most, though, was the pony had both a pointy horn, floofy wings, and earthy strength; she was an alicorn. The first alicorn Smart Cookie had ever seen.

It was then and there than Smart Cookie asked Celestia if she would rule over the land as its princess, if she could get permission. And she said yes! After a few days, Smart Cookie was able to consult with the three leaders, and they agreed—this alicorn was perfect for the job. She fit all of their criteria. And so they let themselves lay their heads to rest, and Celestia assumed the throne. 

Of course, there's a tale to tell about Luna and where she comes in, but that's a tale for another day.

[1] It should be noted that at this time Princess Platinum made a very untimely joke about having her line, the House of Platinum, just taking over the ruling duties of Equestria. It is assumed this was a joke in the hopes that Princess Platinum doesn't actually think this.