In order to reach the masses, advertisers must run ads in a great many venues. Verizon, for example, may advertise in the Times the same day it advertises in USA Today , The Wall Street Journal and other major dailies around the country. Publicity, of course, is most often confined in terms of reach. Such exclusives suggest enterprise on the part of a journalist and boost his or her credit and ultimately, pay check. This refers to how many times the ad is run.
Advertising is a medium that only works with repetition.
Repetition breeds familiarity, name recognition, and ultimately, hopefully, sales. So ads are repeated. Publicity not only doesn't normally get repeated, if the story runs once, it is considered a great victory. In fact, as opposed to advertising, publicity can't control anything. First, it's not free but it is nominal.