He retired because the Bucks paid big bucks for WRs and they would not use him much. Those are his exact words.
https://www.espn.com/nfl/story/_/id/35243913/wr-cole-beasley-unretire-sign-bills-practice-squad
Beasley is the key to longer drives and more touchdowns. Diggs will be in single Coverage and Davis will be wide open. This is a huge addition in December.