Why did Nakamura resign?

I am fairly new to chess and I try to learn a lot by looking into GM games. The Candidates Tournement in Moscow is a good occasion to keep track on what the best chess players are doing. 

On March 20th Fabiano Caruana played Hikaru Nakamura and the letter resigned in this position after Caruana played c3-c4. I fully understand that Caruana has a pretty good tactical advantage that would translate also into material advantage aster white would take the black bishp on h4. But still, why did Nakamura resign? I feel this game was not really over, but I am certainly wrong. Can somebody explain why Nakamura resigned?