In a nutshell, because political speech is so sacrosanct, the Supreme Court has found a right to lie on all sorts of matters. While not having to filter out lies and broken promises would be nice, the court seems more concerned about chilling free speech.
The primary case in this regard is United States v. Alvarez which is sometimes referred to as the "Stolen Valor" case. In this case, the United States Congress passed the Stolen Valor Act of 2005, (18 U.S.C. § 704), which made it a criminal offense to lie about medals earned in the U.S. Military. Alvarez, running for Water Board, claimed to have earned the Congressional Medal Honor. The decision basically boiled down to a lack of harm. Wikipedia summarizes it as follows:
When balanced against the Government's need to protect the value of the Medal, the plurality said that "the link between the Government's interest in protecting the integrity of the military honors system and the Act's restriction on the false claims of liars like respondent has not been shown." Additionally, Kennedy wrote that 'counter-speech' was a sufficient solution to the problem: "It is a fair assumption that any true holders of the Medal who had heard of Alvarez's false claims would have been fully vindicated by the community's expression of outrage... Truth needs neither handcuffs nor a badge for its vindication." 
In the most recent term, Ohio's Susan B. Anthony List - a pro-Life organization in Ohio, furthered the "right to lie" in its case SBA List v. Driehaus. Here, Ohio wanted to make it a crime to do political advertising that contained factual errors. SBA countered that the law would have a chilling effect on political discourse.
The Court addressed this question directly in Susan B. Anthony yesterday. Justice Thomas wrote this for the unanimous Court – referring to both standards in a single sentence:
An allegation of future injury may suffice if the threatened injury is “certainly impending,” or there is a “‘substantial risk’ that the harm will occur.” Clapper, 568 U. S., at _, _, n. 5 (slip op., at 10, 15, n. 5) . . . .
This sentence appears to establish a disjunctive test – i.e., that it is sufficient for the plaintiff to prove either that the threatened injury is “certainly impending” or that there is a “substantial risk” of the injury. Of course, if that is right, then the plaintiff’s de facto burden would be to prove only a “substantial risk” of injury, because there is always such a “substantial risk” of injury in any case of “certainly impending” harm.
Free speech is very important in the US, and a side effect of that is the right to lie. The risk of curtailing lies is obvious, the obvious harm prevented by a law requiring consistency is not sufficient to overwhelm it.