tl;dr> First mover's advantage.
Heavier-than-air flight was, inarguably, pioneered by the United States of America. While inventors from several nations tried, the truth is that the Wright Flyer was the first commercially successful entrant into the field, and the most major advances of the early industry were, indisputably, American. That coupled with the fact that the first international destinations were to countries still using the Imperial system (to wit much of Europe) meant that a custom of using customary units was strongly entrenched before the metric system came into major prominence.
Internationalization typically proceeds not by fiat, but by custom. As a result, American measurements within the device and external to the device, were initially English measurement. As regulation evolved, the custom stuck in order to avoid confusion - as evidenced by the fact that when a Russian pilot lands at a Russian airport, the conversation is still held (at least in theory) in English. (Note: This is the regulation - not always the actual practice)
Put another way, when Lindbergh first crossed the Atlantic, his charts were American. When flights began to regularly cross the Atlantic, some standardization needed to occur. Being the originator of the device, Americans were at an advantage in negotiations when declaring what the standard would be. Having declared a standard, however, there was no compelling reason to change it.
Most standards evolve in this way - the first to market has an advantage that is only overcome if there is a sufficiently compelling reason to do so. We all know HTTP is insecure - but we all know everyone can speak it. So, even if a better standard exists, it will continue.
Robert P. Crease talks about this in his book "World in the Balance: The Historical Quest for an Absolute System of Measurement". He relates the reason the United States initially adopted the English system of measurement as follows:
If machine parts had to be changed, or if the dimensions of most of England’s exports had to be altered, untold confusion would result in the national economy and the entire nation would be thrown into a recession or a depression. (Similar arguments have been presented by anti-metric spokesmen in the United States throughout the present century.
Indeed, the United States is thus no more "imposing its system" on other countries, then the French did on the United States many years ago. Crease writes of the adoption of the metric system as follows:
Dombey’s meter and kilogram were purchased by someone who sent them to a French official in Philadelphia. This official put them into the hands of someone who, not realizing their significance, never conveyed them to the U.S. Congress. Had Dombey’s mission succeeded, it might have provided momentum to advance the metric system in the United States. “The sight of those two copper objects,” Linklater writes, “so easily copied and sent out to every state in the Union, together with the weighty scientific arguments supporting them, might well have clarified the minds of senators and representatives alike
The metric system is inarguably simpler than the English system. Pure decimalization makes the math easier - but there is custom to overcome. Indeed, the very units you would have the international system use - namely the meter, which in turn is derived from the second, a unit of time, had to ultimately be derived from its reference to London. While Paris had the 'meter stick,' it was turned into a unit of time (namely the amount of time it took for a pendulum to swing) which, in turn, came from London. The kilogram - the only remaining unit of measure still judged against a physical device - is based on a French artifact.
The point with all of this is this - there is no truly non-nationalistic unit of measure. Indeed, reading through Crease's book, you find how odd it is that we settled on any standard. To attribute some imperial motive (no pun intended) is silly, though, because it merely pushes the envelope back.
You are arguing for a French unit. The United States is asking for a compelling reason to change the convention. Both are nationalistic. Neither is "pure"