Two dark headcanons and a nicer one.
1) Crystal ponies eat meat. They don’t have livestock, but they do have the mindset that any animal that attacks them is fair game for food if the ponies survive and bring it back. They also can’t venture in the wilds without being attacked by everything that moves, so they don’t even have (or need) hunters to have a healthy amount of meat available at all times back home.
This mindset led to a very, very awkward first contact with yaks way back then, when crystal ponies couldn’t tell bandits apart from animals because of the yak ‘language’ amounting to screaming and smashing stuff.
2) Pegasi have the highest birthrate in Equestria but are the least numerous. The other two tribes do their best to never think about why, assuming they’re even aware of it.
Turns out that pegasi go on one last flight when their wings start failing, unless they have something else to cling to in life, like grandchildren without parents and aunts/uncles to care for them, non-pegasus spouses who wouldn’t get it
or an ‘unbroken’ academy record. They don’t handle losing their wings well either.
There are exceptions like Fluttershy, and those make up the majority of pegasi after the population bottleneck of G5’s loss of magic.
3) The Crystal Empire exists in the Equestria Girls world. Complete with crystal ponies. That’s why no one is surprised or bothered with Princess Twilight being a pony, though they do find the alternate universe thing weird.
3b) Alternatively, they’re anthro crystal ponies. And EqG!Rainbow Dash has the entire pinups collection of Crystal Mares.