The reasons for not designing to the metric standard of measurement are dwindling.
The U.S. is a multilingual country, in its measurement units as well as its language. The rest of the world, however, uses one language for measurement, and that is metric. This makes the U.S. the only industrialized country that has not standardized on this system. And this presents a few problems.
For example, in 1999, NASA engineers launched their Mars Climate Orbiter, a $125 million spacecraft designed to explore the surface of the red planet. For nine months, engineers monitored the spacecraft's flight and altered its trajectory as needed. The engineers knew that two crucial programs spoke in different units of measure. However, the conversion algorithm that was to address this difference was accidentally omitted. The Climate Orbiter either flew too low and crashed into the planet or it glanced off into outer space. Either way, they have not heard from it since.
This is not an isolated example, simply a reported one.
The U.S. system is a hodgepodge of units, where names can apply to unrelated units. One example is a unit called "ton." There's short ton, displacement ton, refrigeration ton, nuclear ton, freight ton, register ton, metric ton, assay ton, and ton of coal, and none are equivalent to each other. According to the U.S. Metric Association, the U.S. chaotic collection of confusing units means Americans don't really understand the quantitative information they encounter. Another problem caused by maintaining a mix of units: No manufacturer wants to have two production lines, one English and one metric. This setup means that tools, machinery and parts must be stored and inventoried separately, increasing the burden on record keeping and warehouse space. All of this costs business money.
A related problem involves overseas customers. Every other country is demanding that more U.S. products be built and labeled to metric standards. Even European Union countries that have long been good U.S. customers no longer want our nonmetric products. In turn, they don't want to supply the U.S. with nonmetric products anymore because of the additional cost.
Then, there's the problem of competitiveness. Companies must sell overseas to stay in business. But as the only holdout for the nonmetric system, U.S. corporations face stiff competition from others already standardized on metric.
The metric system offers benefits. For one, thinking in metric units allows a company to communicate with the rest of the world without hindrance, a fact such high-tech industries as semiconductor and medical know well. Plus, use of metric implies greater technical sophistication.
People avoid using metric because of familiarity with inches and pounds, and the confusion that comes from converting back and forth between the systems. Until people can build a similar familiarity with metric, reluctance to use it may continue.
With laws in place, and with most engineers somewhat familiar with metric, the only obstacles left to overcome is a lack of component availability and our resistance to change. The first obstacle is being overcome with the increasing number of metric products. The rest is up to engineers.