Discord nailed it.

It’s like Mario and Luigi’s full names being “Mario Mario” and “Luigi Mario”

I been callin’ her Applejack Sweet Apple since Season 2.

@The Smiling Pony
To clarify, mass migrations to different already established nations. Migrating from an inhabited land to an uninhabited one wouldn’t require something like what happened in europe and several other parts of the world because they were moving to places where they had to establish themselves. The previous nations were obviously very close to each other if not right on top of each other, so they’d know who went where and there’s no one in the new locations for them to clarify their place of origin to.

Gen 5’s migration shift would entail them moving to a place that ponies already inhabited, just different towns and in such mass that they’d need to differentiate themselves from others who might already live there historically and be established, as opposed to the original 3 tribes who would be moving to a place with no ponies to establish themselves to.

@Background Pony #2E85
Did you… did you miss the episode with the windigos, the never ending winter, the mass starvations?

MLPverse never had something akin to the Black Plague that forced mass migration and thus a need for an easier way to distinguish someone’s ancestral roots instead of their general location, that’s why they don’t have surnames. Applejack is just Applejack, there’s no Applejack Smith originally from Foalington who needs to be distinguished from her after they moved following the mass death of crop growers and other lower class positions that needed filled in Ponyville. Presumably this is why the Sparkle family doesn’t have a Sparkle except Twilight and why the Pie family adults doesn’t have anyone with the surname Pie. Instead it’s Velvet, Quartz, you get it. Secondary names are treated no differently than single names because they’re not surnames passed down, just a part of their singular name that happens to have two words in it ala Native American “running horse” or the like.

Gen 5 is presumably after a mass cataclysm which is why they’d have to adopt surnames in order to distinguish their original roots after moving from other places across Equestria, explaining both the new surname and their drastically reduced population.

SEAsia ordering, obviously. Apple Applejack.