The answer is pretty simple: we are desperately in need for more spectrum (at least in urban areas), licensed and unlicensed, for wireless internet traffic. And there really are no unused bands that can simply be reallocated for this purpose without dealing with the users already in the band.

Although airport RADARs sound like a poor choice of a service to share spectrum with, they have the benefit of being stationary, not very numerous, and typically located far from the urban cores where the spectrum is needed most.