It is simple....
If you don't win, how can you say your input was the most valuable?
Maybe it would have been more valuable if you shot less and got your teammates going more? Maybe it would have been more valuable if you defended Curry instead of Livingston? Scoring 40/13/8 is great, but it didn't make your team win, so it was obviously not what the team needed to win.
Iggy on the other hand did EXACTLY what the Warriors needed him to do in order to get the win. Therefore is input was more valuable.
If you don't win, how can you say your input was the most valuable?
Maybe it would have been more valuable if you shot less and got your teammates going more? Maybe it would have been more valuable if you defended Curry instead of Livingston? Scoring 40/13/8 is great, but it didn't make your team win, so it was obviously not what the team needed to win.
Iggy on the other hand did EXACTLY what the Warriors needed him to do in order to get the win. Therefore is input was more valuable.