After you Kill Mallyx

I thought after we beated city, veil, gloom and foundry we would be allowed free at the citadel but istead of it i got to see the quest to go by the 4 places to then get in the citadel again, is that the only way to get there? Or i'd be able to if i didn't got the reward from Zellyx? Thanks Very Much for your answer

Once you kill Mallyx, you have to do the entirety of the DoA over again in order to fight him once more.

I have a feeling this was done to keep his greens relatively rare, though I've no idea the actual rationale.

searching for any rationale in regards to DOA is an exercise in futility