So, simply put, The Collector didn’t fit into the narrative of Guardians of the Galaxy Vol. 2 and that is why he isn’t in the movie.
Guardians of the Galaxy Vol. 2 is written and directed by James Gunn with a release date set for May 4.
The Collector wasn’t right for the story of Guardians of the Galaxy Vol. 2 because, although it’s an incredibly enormous film, it’s a much smaller film in certain ways with few characters.
Even though Benicio Del Toro won’t be back as The Collector in the Guardians of the Galaxy sequel, he is almost surely going to be back in the MCU at some point.