tl;dr: the US has a long and tragic history of half-hearted attempts to switch to metric, beginning in the mid-1800s (when metric was made an international standard, with the active participation of the US... one of the original seventeen signatory nations to the Metre Convention), and continuing on through today, where the US is one of three remaining countries (with Myanmar and Liberia) that has not adopted the International System of Units (SI) metric system as their official system of weights and measures.

Along the way we get sad stories like the loss of the Mars Climate Orbiter in 1998, where everyone except Lockheed Martin was using metric units in the parts and software written for the orbiter.

So come on, USA! Keep trying!