The Flash "The Once and Future Flash" S3E19 (4/25/17) SPOILERS!

Okay, I'm not fully up on the ins and outs of time travel, but why couldn't Barry from 2020, the year the fizzycist built the speed force trap, go back to 2017 and save Iris? Is it because he has a bad case of mopery in the future?
Or, maybe the moral of the story is, Iris is gonna die. It can't be changed, it will happen, it has happened. So, pick yourself up, leather boy. Keep going. Don't be such a whiny ***** in the future.
Any bets on who Savitar will be?

The Flash "The Once and Future Flash" S3E19 (4/25/17) SPOILERS!