The narrow legal answer is: false advertising regulations generally only apply to commercial trade which doesn't include elections; wider fraud offences are (unlike false advertising) not strict liability so would require you to prove that they knew the representation was false at the time it was made (as opposed to just changing their mind later), which is difficult, and in any case it's unclear that a court would consider a vote to be a gain of money/property; there's no unilateral contract here as there's no objective intent to create binding legal relations with every voter in the country; and administrative law concepts of legitimate expectations don't apply to manifesto/pre-election political promises per R (Wheeler) v PM. (That's all for English law, but I imagine the concepts are similar in the US).
The wider answer is: laws are made by politicians. They're not going to pass laws that would make them liable for lying to win elections.
The narrow legal answer is: false advertising regulations generally only apply to commercial trade which doesn't include elections; wider fraud offences are (unlike false advertising) not strict liability so would require you to prove that they knew the representation was false at the time it was made (as opposed to just changing their mind later), which is difficult, and in any case it's unclear that a court would consider a vote to be a gain of money/property; there's no unilateral contract here as there's no objective intent to create binding legal relations with every voter in the country; and administrative law concepts of legitimate expectations don't apply to manifesto/pre-election political promises per R (Wheeler) v PM. (That's all for English law, but I imagine the concepts are similar in the US).
The wider answer is: laws are made by politicians. They're not going to pass laws that would make them liable for lying to win elections.