In the late 1700s, Thomas Jefferson wanted the United States to adopt a unified system of measurement and saw the metric system as the best solution. However, a pirate attack in the Caribbean disrupted these plans. Joseph Dombey, a French scientist carrying a kilogram and meter stick to demonstrate the metric system, was captured by pirates. By the time France sent another scientist to explain the system to the Americans, Jefferson was no longer in office, and plans to go metric were disregarded.
One of the ongoing goals in science is to reference all metric units to fundamental forces. Basically, we want a system where you can write down everything you need to recreate all our measurement systems.
Right now, most are referenced that way, but not all. Last I heard the kg was being difficult. I believe the plan is to reference it to a perfect sphere of perfectly crystalline silicon-28 of a given size. Creating such a sphere is extremely difficult however.