In part because it does not meet the Frye standard.  And some Judges and Lawyers are required to undergo polygraph examinations.

In Frye v. United States, [FN24] the Court of Appeals of the District of Columbia established the standard for the admissibility of novel scientific evidence in federal courts??the general acceptance standard. [FN25] Under this test, any novel form of scientific evidence admitted into evidence "must be sufficiently established to have gained general acceptance in the particular field in which it belongs." [FN26] This test requires the trial judge to determine, based on expert testimony, whether legitimate debate exists within the scientific community regarding the reliability of the particular technology. [FN27] Although the true meaning of general acceptance is somewhat unclear, [FN28] it does not require a showing of universal acceptance. [FN29] In California, subsequent cases have delineated that general acceptance means a "consensus," or the absence of public opposition by ".scientists significant either in number or expertise."'

http://www.tylerlaw.com/pepperdine.rtf

In other words, with respect to polygraphy and its over 40 years worth of being "established," it has never met, nor will it ever in its current form, generally accepted scientific standards.