Other plausible candidate explanations, though without data. They're all testable though:
- Higher opportunity costs for high IQ people since the 80s.
- Greater cultural disparaging of the military in elite circles, so it is not aspired to by those of higher ability.
- Decreasing trust in that military is a force for good or really much needed (see the decline as the Soviets turned friendlier under Gorbachev, the levelling off and rise around Gulf War I, and the resumed decline after that).
- Fewer smart but lower income kids needing to use ROTC to afford college with expansions in student aid.
I'd bet on the first and last being most important.