Aether's popularity just goes back to Gilgamesh being advertised as the subreddit server during ARR's launch, and a lot of the raiding community started up characters and Free Companies there early on. You likely saw similar population spikes on Ragnarok/Chaos back when Angered was actively involved in world progression.

This is fairly standard behavior in MMOs. Players congregate where there's action, so high population servers get bigger and low population servers get smaller. Cross-server PF and statics make your server choice less important, but you won't truly move towards a stable equilibrium without Cross-DC PF. You either live in the big city or you commute.