Why has the United States historically demonstrated such a high degree of inconsistency in its commitment to gifted education?

It is hard to determine who is truly "gifted," as there are different types of intelligence. A gifted athlete may lack skills in fields that are normally measured on state-mandated standardized tests, for example. Since much of education is considered in terms of what can be measured on these tests, the athlete may not be given an outlet in which to harness their talents.
American public education is often geared toward members of the mainstream public, who are assumed to be of average intelligence. The students who have an IQ of 100 (the average) are supposed to benefit the most from classroom instruction. Those who are too far removed from this measurement do not benefit as much as those who are around average. Those students who test lower than their peers on reading and math assessments are typically given outside help with Individual Education Plans (IEPs) in order to ensure their success. Many school districts lack the funding for gifted education, and it is just assumed that these students will do well in the public school setting without any outside help. While it is true that students who are gifted in verbal and numerical intelligence do well on standardized tests, many of them become bored with instruction. Their talents are not cultivated in a mainstream classroom. Instead of academically growing, the students may not be encouraged. These students may suffer when they go on to higher education as they do not develop the study habits used by their peers in high school.
While there are many individualized programs for intellectually gifted students, American funding for education has not been adequate to serve all of their needs. A great deal of public education is tied to a class's score on standardized tests. It is assumed that students who are gifted in verbal and numerical intelligence will do well on these tests, and these students have historically been underdeveloped in the public K–12 school system.