More attemps, but lost loot

I went to kill Olroth (expedition boss).
Had more attemps yet, not really did this mechanic this season before.
First try, the boss died, the loot dropped. I was inattentive a little, because the boss just died, but one new laser started, and killed me.
Went back for my loot, but the boss still departed, but my loot also gone.
Why we have more attemps, if we can't go back for the loot ?