Reading the DoL's advisory on this [1] suggests that it all hinges on the concept of blue-collar vs. white-collar or production vs. staff employees. I can only imagine that the 'production' employees are seen as more interchangeable and largely incapable of having multiplicative effects on the company's business model. A programmer or accountant who's getting a raw deal can find a better job (where better means pay commensurate with contributions), but a tier 1 help desk tech or factory worker will likely have a lower ceiling on possible pay to begin with, and (more) trouble finding a way to secure compensation that scales with corporate success.
It is surprisingly hard to find explanations for this - the web is choked with articles explaining who's exempt and who's not, but not so many with justifications or reasoning or historical context.