Unicorn magical power is enhanced by orgasms. This is why all the strongest unicorns we see are nerdy girls: they're fucking weirdos in the bedroom, but also, they've learned to channel the very frequent orgasms they give themselves into their magic reserves.

Star-Swirl the Bearded discovered this unintentionally by just being that much of a gigachad (most unicorns have a little bit of him in their ancestry, like Genghis Khan in our world). The other five unicorns who'd raise the sun and moon with him before Celestia and Luna showed up were his mare harem, but even he couldn't satisfy six mares enough on his own to make it easy for them all as a group.

Celestia and Luna have the power to raise the heavenly bodies all by themselves because of the frequent orgies they have with colts and fillies from around Equestria. Only younger ponies have the stamina needed to give the princesses the sexual energy they require to keep the day-night cycle going. Celestia's trying to tone it down to a small group of royal consorts now that Luna's back to split the effort.