SWTOR confirms via Instagram, where they seem to be the most active, that Relics of the Gree is having problems with scaling to level 60 so it won't be coming back just yet.
joshmcgoo Why isn't the gree event up?
swtorofficial @joshmcgoo not happening right now.
swtorofficial @joshmcgoo yeah.. turns out scaling it to level 60 caused a lot of issues and we decided not to turn it on right now rather than providing a less than awesome experience.
whats_demuresoul So it's getting postponed or...?
swtorofficial @whats_demuresoul yeah. It'll be back at some point in the future.