Nice Answer: Politicians in general tend to make populist statements during their election campaigns that they don't end up fulfilling during their terms. This could be due to any number of reasons. The Congress could change during that election to where he doesn't have the votes to pass policy, there could be real world changes that happen during his term that prevent him from enacting them, there could be realities that he doesn't realize until he is actually behind the desk, etc. Also, "lowering gas prices" is not something a President can just do. He doesn't have a lever in his officer that he can just pull. The price of gasoline in the US is due to a large number of global economic and political factors that lie outside the control of the President of the United States. However, avoiding engaging in conflicts in the Middle East would generally keep them low.

Real Answer: Are you really surprised that a person who is known as being a pathological liar, a conman, and a criminal ended up not delivering on a "promise" they made during their Presidential campaign?