Because of Djaq, and because he put his pissing contest with Gisborne ahead of her life and safety.

Up until that point, I really bought into Robin's message of love and charity. But in that episode, it became clear to me that Robin was just a good soldier for King Richard, and I was done with him. Djaq forgave him, but I'm not sure I could. ;)

(Yes, I'm maybe just a bit over-invested in this fandom, lol!)