Being able to use magic does not mean one will be good at it, likewise, being able to fly does not mean one would be able to/be good at it. Plus, having wings means extra muscles to strain or bones to break, and having to work around wings when it comes to daily activities, same for horn really, that would be killer (literally) if you sleep with someone else, plus if you’re bad at magic, you could be downright dangerous to those around you. Earth ponies don’t have to worry about wings getting in the way, or potential magical disasters.
Despite all that, I’d probably be a Pegasus if I could choose, or a Unicorn, I would not pick Earth Pony, but that’s mainly as I’m used to dealing with handicaps.