Opponents have argued that the short-term cost of converting is too high, it would undo our culture, we don't want to, etc. I'm not going to go into details here.
However, I have had the distinct sense that something has been missing from all this debate. I think the main cause of not metricating is simpler, and goes back to why the United States has been so slow to adopt social programs such as universal health care and other such programs: White supremacy.
Of course, you may be asking, what does this have to do with race? I'd say it has everything to do with race. I came across this interesting paragraph on teammetric.org on American students' poor grasp of measurement:
Stated plainly, measurement is “the domain of least relative competence for U.S. students” (Barrett 2012). This finding is supported at the district, county, and state levels. In the U.S., weights and measures are generally learned in the study of spatial measurement (Smith 2012). Extensive evidence has shown, and continues to show, that U.S. students’ grasp of spatial measurement—length, area, and volume—is poor, despite the wealth of spatial experience and knowledge they develop and use outside of school. This evidence includes analyses from the National Assessment of Educational Progress (NAEP) of performance by 4th, 8th, and 12th graders (e.g., Blume, Galindo, & Walcott, 2007); cross-national comparisons such as TIMSS (National Center of Education Statistics, 1997); and smaller research studies that have focused on students’ patterns of reasoning, e.g., studies indicating that students often confuse area and perimeter (Chappell & Thompson, 1999; Woodward & Byrd, 1983). Where the NAEP results show low performance in the entire U.S. population, performance is weakest for low-income and minority students, who lag further behind white students in measurement than in any other content area (Lubienski & Crockett, 2007).http://www.teammetric.org/measurement/
To me, the most interesting sentence of all is the last one: Students of color lag further behind white students than they do in any other subject, and this is a subject on which American students as a whole do poorly. It appears that people of color are being hurt by this policy of not metricating more than white people are. Think also of the economic effects. Historically, who has born the brunt of poor economic planning in this country? People of color. Thanks to this, white people have generally been able to escape the consequences of such bad decision-making. And who is it that dominates the discussions on metrication? White people. For the proponents, this question often seems like little more than an intellectual exercise engaged in by hobbyists, while for opponents, not converting has become a point of pride for this country, a belief that we are special as a nation. They can afford to argue this issue in such a detached manner because they are not so negatively affected.
The same has been true with health care and a number of other social programs. This is something that has been built into this country for centuries, the elite dividing and conquering the working class on the subject of race, the divide hampering much economic progress in this country.
I suggest that people of color begin to find their voice on this issue and discuss their experiences with our weights and measures and failure to metricate; plus, their experiences with receiving a poor education in measurement, as shown above.
EDIT: TL;DR: Basically, white people are buffeted from the negative effects of bad economic policies, including the policy of not metricating, causing them not to see the need to change. People of color bear the brunt of such negative effects, but on the subject of metrication in the U.S. discussion has been dominated by white people, largely middle-class white people. In addition, there is evidence that students of color lag behind white students in measurement more than in any other area of education, and American students as a whole lag the rest of the world more in measurement than in any other area; failure to metricate likely contributes, because teaching metric units only is far more straightforward than teaching dual units or only customary (as that is a complex collection of units), such straightforwardness saving classroom time and allowing students to get to actual practice. I hope people of color can find their voice on this issue and speak out about how these policies affect them.