Because it lowers the threshold for a total informational compromise attack from "exfiltrate 34PB of data from secure govt infrastructure" down to "exfiltrate 100KB of key material". You can get that out over a few days just by pulsing any LED visible from outside an air-gapped facility.
There are all sorts of crazy ways of getting data out of even air-gapped machines, providing you are willing to accept extremely low data rates to overcome attenuation. Even with million-to-one signal-to-noise ratio, you can get significant amounts of key data out in a few weeks.
Jiggling disk heads, modulating fan rates, increasing and decreasing power draw... all are potential information leaks.
