Kershaw pitched Game 1, and then Game 5. If Roberts wanted Kershaw available to start a possible Game 7, then they should have started him in Game 4.
He pitched 4 innings in Game 7, which is probably all he had in the tank after pitching Game 5.
And FWIW, if Kershaw was the magical answer, then why did he not win Game 5 when he had a 3-run lead?
Darvish has been a great pitcher for the Dodgers this year. Of the 2 starters for Game 7, he seemed to have the better track record. For whatever reason, he really was a mess in Game 7. Reportedly, he didn't look sharp in warmups.
Yet for all the discussion of why not Kershaw, or why Darvish, or any other questions about the pitching in Game 7, the bottom line was that the Dodgers didn't hit all that well in the WS, and stranded runners on base throughout Game 7.