They did. And you did.
The "Daisy" ad was aired only once...as a paid ad. On September 7, 1964 (NBC Monday Night at the Movies, David & Bathsheba).
A great hue and cry followed and the DNC cancelled any further exposure of the ad.
However, the networks decided that the ad -- and the response to it -- was newsworthy in and of itself. As a consequence, they ran it interminably on news & analysis shows for the next month.
By the end of September, over 95% of America had seen the ad an average of 4.2 times. It is not known, for sure, whether this process had been pre-agreed to by the DNC and the networks -- but nobody would be shocked if it had.
But when the prospect of the election of a candidate like Goldwater who would try to shake the politicians out of their love for increasingly, large government, the libs all banded together to destroy him, Goldwater.