Aparentemente, porque o ator (Scott Caan) pediu um papel reduzido. Veja sua resposta para isso:
"I do five episodes less than [series star] Alex [O’Loughlin]," said Caan, who lives in Los Angeles while the show shoots in Hawaii. "I just do a couple episodes less so I can come home to see my family and stuff. They were cool about it so we just all came to that."
Outra fonte elabora:
Unlike some of the show’s stars and guests who have made themselves at home in Hawaii, Caan continues to live in Los Angeles. [...] Caan has a daughter born in July 2014 and activities outside “Five-0” including a play he wrote, “The Trouble We Come From,” which was performed in Burbank in 2015.
Então parece que Caan está muito ocupado com outras coisas e prefere viver em Los Angeles ao invés do Havaí (na verdade ele se meteu em problemas depois de expressando essa preferência ).