The standardized systems of units appear during the industrial age. An early hurdle for manufacturing was that nobody really agreed on what sizes things should be. A bolt from one machine might be radically different from a bolt on another machine, even when they came from the same manufacturer. By standardizing units, it was much easier to machine exact copies of parts and assembly was massively simplified. The French advent of the blueprint made this even easier, and as a final step, a set of standardized tool sizes were implemented so that everyone had the same size bolts, screws and nuts. This way, even if a factory far away made the nuts and bolts for my machine, I'd know they could fit. During the industrial revolution, Britain was at the forefront of this standardization movement.
Without standardization, only a small amount of those things in the box would have fit
At very nearly the same time, a similar, but completely separate standardization movement was occurring in the United States. Since the US was still quite inaccessible from Europe, it was largely self sufficient. Almost everything in America was made in America by American laborers using American natural resources. The same need for standardization existed as in Europe, but with very little industry traveling back and forth over the Atlantic, the two efforts never really touched base with each other. The US system thus ended up being completely separate and unique (though similar to the Imperial system as it's largely based on the British units). So while Europe was eventually faced with finding a system everyone could agree upon, and eventually settled for the Metric system; America just needed to accomodate for itself. When the Metric system eventually did reach across the pond, the US system was cemented in place and not ready to move.
This is the big issue with switching to the Metric system: It would involve changing every nut, every bolt and every screw on every machine made in the US to a different size. Just like when the standardization of the US system first fell into place, every manufacturing plant and factory in the US would have to change how they did things, and that's a costly and difficult change to make when there's already a functional system in place. Further more, anything manufactured before this change could no longer get spare parts that fit.
Americans would have to throw your tools out if the US went metric
Obviously there's also the social issue. If you're used to one system, you probably don't want to learn another one. It's really not that hard to adjust, but it will undoubtedly cause a bit of confusion, and more importantly, resistance – any time I see an article relating to switching the US to the metric system, there's a slew of angry comments resisting it. As I mentioned before, I can understand this; you use these units every day, and it's almost like being asked to throw away part of yourself.
Curiously, the truth is that the US is slowly becoming metricated, and perhaps slow and steady is the way to do it. Machined parts may not use the metric system and it may not have much of a place in everyday life, but the US government officially adopted it for military and government use decades ago. Certain stretches of road use metric as well, most notably Interstate 19 in Arizona, where distance measurements are almost entirely in Metric. Even in the consumer market, there are a few places where metric units pop up. The two-liter soda bottle is one example. Dental floss sold in 25 or 50 meter packs is another. In fact, a 1992 amendment to the Fair Packaging and Labeling Act require federally regulated "consumer commodities" to have measurements in both metric and US standard units. So perhaps change is coming after all…