There is the cynical answer, but there's also the fact that the US equipment is actually pretty damn good and can be delivered in large quantities, though it it usually more expensive. You also have to consider alliances, if you buy equipment from country X then that country will be able to provide spares/ammunition/trained personnel if shit hits the fan. And the US is not blocking allied countries from purchasing equipment elsewhere or from developing their own industries. In fact it is usually angry at countries like Germany for not pulling their weight in domestic production.
Military sales are more politics than business. The French behave much like the US, they'll buy foreign kit only as a very last resort. It is a strategic decision that a country has to make based on manufacturing capabilities, ability to generate orders (domestic and foreign) and the planned purpose for their military forces.
