A wall on the US/Mexico border has been a cornerstone of Trump's platform throughout his campaign for the presidency. Shortly after declaring that, if elected president, he would build a wall to keep out Mexican immigrants, he was asked who would pay for what would surely be a very expensive structure. His claim all along has been that he would get Mexico to pay for the wall, without offering specifics as to how he would get Mexico to pay for it. When he finally met with the Mexican President Enrique Pena Nieto, he did not bring up the topic of Mexico paying for the wall. It was a cowardly move that showed he is not the serious or aggressive negotiator he claims to be.
Yes, Trump chickened out in not telling Pena that Mexico would pay for the wall. In general, Donald Trump is rude, offensive, and racist. Him doing something like this does not surprise me. Him doing something like this is exactly what I would expect from someone who is clearly not fit to run for president.
Trump knows what he is doing and is a master manipulator. He did not say anything for a reason, perhaps partly because Mexico has denied that they said they would pay for the wall. It is difficult to know the truth or the motive behind Trump's actions because he is such a manipulative person in both business and politics.
Trump did not mention it because he knows that it was an empty promise and that it was not going to happen. It would have been embarrassing for him if he had brought it up, and until the President of Mexico brought it up, he could continue to make the promise that they would.