CypherHoof

Well, you can't choose who you fall in love with. James may have been a bit of an ass, but Lilly was the Princess Cadence of her world

And his father's. His mother (and Snape) had a doe....

Well, some sort of spirit for sure. Here is an explaination from later in the strip

As much as I would love magic or flight - Earth pony is the best fit for who I am.

I think it's a division of labour sorta thing. The unicorns do magic, because only they can; the pegasi use wingpower to fly, manage the weather and so forth, while the earth ponies do the ground based tasks like farming and baking. It's obviously not a rigid caste system (as Fluttershy is doing a task you would think an Earth pony would be assigned) but presumably it is frowned upon - look at Mrs Cake's reaction to Starlight whipping up a cake, for example. At the end of the day, everypony has a role that they can do comfortably, feel valued for their service, and make a few bits in the process. That seems a worthy goal for Pony society as a whole. Also worth thinking that, as Twi says, very very few unicorns can actually do more than a few spells aligned with their cutie mark; maybe there just aren't that many unicorns that CAN fly with magic, that they would displace pegasus ponies from that role.