Because he didn't profit from the cryptocurrency, he was just a stupid guy who was trying to short the market.

What's funny is that the Winklevoss twins challenged Jamie Dimon to short BTC if he truly feels that it's a fraud. It hasn't been done (more than likely due to speculation that it's) because Dimon probably doesn't truly feel that Bitcoin will fail.
Tyler called the JPMorgan Chase CEOs criticism of bitcoin cheap and added, We encourage Jamie Dimon, we encourage him to personally bet against it, bitcoin, take J.P. Morgan's balance sheet, bet against bitcoin. We'll see what happens.
Check this out to learn more about this challenge:
http://www.foxbusiness.com/markets/2017/12/15/bitcoin-battle-winklevoss-twins-vs-jpmorgan-ceo-jamie-dimon.html