You need an airship to get there.
We needed an airship to get to Praetorium, the Howling Eye, and probably some other places I'm not remembering too, so what's your point?

They eventually realized needing to bring a full party to Coerthas Central or East Thanalan was a pain for the chimera/hydra fights, so ideally they would realize the same thing for Diadem and make it freely queueable. Hardly the biggest problem that plagues Diadem right now, but you know, baby steps...