If a presidential candidate did not follow through on his promises then they would not get the public support that they need to run for office. Following through on your promises, at least in the public and medias eye is extremely important in your campaign. One wrong move and your campaign could be history.
Presidential promises is a part of the mandate inclusive of campaign strategy to get the American people to vote for his constituency. In that regards, a presidential candidate strategy for getting elected is promises that the policies of the previous administration will not get repeated. That is how Obama won.