If the so-called "legit national advertisers" (and who says companies other than Coke, Pepsi, and GM aren't legit?) don't advertise on radio anymore and took their "somewhat entertaining ads" with them, I say it is the fault of radio listeners who defected to other media and made radio less attractive to those companies.
In other words, there should be more looking in the mirror by those who ask "how did this happen?"
No it isn't - it's a fault of the medium that no one can help. The 'big' advertisers all work through their agencies with big multi/omni channel marketing programs that are digital first focused with near instant analytics - in that they want the analytics and reporting and the ROI to tie out to nearly every dollar spent. FM radio doesn't fit that model no matter how many programs and sales pitches, PPM data and never will. Analog mediums just don't compare unless you are throwing dollars at general awareness campaigns but when incremental dollars come up they go to other places first. That's not to say radio doesn't have value, but not in the way it once did.