is the air density the factor ?
Yup. YAL-1A showed that we could put useful devices up into higher alts...which is why it was cancelled.
Flying a big COIL laser at 40,000 feet alt is silly (workable, but 50km range, toxic chems, etc.).
Flying an unmanned drone at 65,000 feet while equipped with an electric laser is, in contrast, more appealing.
She’ll be airborne in 2021 or close to it.