Fridge Horror: Snowdrop, according to Luna, was the only mortal pony who ever truly understood the night. It's heavily implied that Luna became Nightmare Moon after Snowdrop's death.
One could argue that Luna became Nightmare Moon (and then sealed into the moon) while Snowdrop was still alive. But for obvious reasons, Snowdrop was no longer alive to greet the reformed princess. Imagine the heartbreak she went through when Luna/Nightmare Moon was banished.
Though for the record, the short itself never states when Snowdrop died (aside from it being a thousand years prior to the main show). It's very possible that she lived to a ripe old age.