Technically, the United States has been using the metric system since 1866, but the reasons for our continued use of non-metric units is complicated.
In 1866, Congress allowed use of the metric system for the first time. The 1893 Mendenhall Order standardized our system of customary units to metric units; the U.S. customary system we use (not the same as Imperial) has been defined in terms of metric units ever since. In fact, we were a signatory to the original Metric Convention of 1875.
Fast forward a century. In 1975, Congress passed the Metric Conversion Act, which provided for the creation of a U.S. Metric Board to support an orderly, voluntary transition of the U.S. to metric. Unfortunately, as the rest of the world made mandatory conversions to metric in the second half of the 20th century, the voluntary nature of our switch led to a general feeling of apathy (and in some cases antipathy) towards a switchover to metric. Bottom line, support for metrication dropped off after the 1970s, and the Metric Board was abolished in 1982.
Of course, not all is bad for metrication supporters. Most science, medicine, and engineering fields use metric extensively or exclusively, and manufacturers have begun listing quantities such as length and volume in both metric and U.S. customary, with some even listing units solely in metric; a 2 liter bottle of Coca-Cola is a prime example of this. Some roads in the U.S. also use metric units alongside customary units, or, in the case of Interstate 19 in southern Arizona, almost exclusively metric1. Most of this was done in preparation for the U.S. Department of Transportation's switchover to metric in the 1990s; legislation prevented that plan from advancing. Also, some legislators believe that now may be the time to move forward on metrication, as a state legislator in Hawaii has introduced legislation that, if passed, would mandate the use of metric units in all Hawaiian public institutions, including public roads, beginning on January 1, 2018.
Now, all that being said, I support a mandatory, but gradual switch of the U.S. to metric, beginning with public institutions. It's easier to use (base ten versus our current system), and it's already used by much of the rest of the world, so trading would be a bit easier. By phasing out all aspects of customary units in public education early, a gradual switch of road signs to metric units, and long-lasting public service announcements detailing the changes, the foundation for a full U.S. adoption of metric would be laid, prompting more private companies and businesses to switch. After about a generation or so, Americans would be able to, in theory, work and live by metric units only. Of course, customary would take a while to die out in this country, and there would be the inevitable private business or noncompliant local or state government that would say no to metrication and engage in a process not too dissimilar to the "massive resistance" policies of the 1950s and 60s.
And no, the NFL wouldn't have too many problems, as 10 m = 10.936 yd. So there's that.
1 As the Arizona DOT installs new signs, however, Interstate 19 is gradually being switched to customary. Speed limit signs were already listed in miles per hour.