In an article about the "meat and potatoes" success of TNT's Rizzoli & Isles, Advertising Age writes that "cable shows tend not to hold audiences for commercials as well as broadcast programs."
A recurring topic is that there are still big premium for broadcast network advertising vs. cable, even on per ratings point basis. The only two reasons I've seen that ever made any sense for continued broadcast premium are the one above, and that overall, on average the audiences of broadcast network shows watch less TV than audiences of cable shows.
Separately the article notes that:
Turner isn't solely in the business of coming up with middlebrow police procedurals. TNT is expected to air a second season of "Men of a Certain Age," a critically acclaimed series featuring Ray Romano, Scott Bakula and Andre Braugher, as well as "Southland," a dark police drama that was jettisoned by NBC. But Mr. Wright suggested a network has to devise a portfolio of programming that is financially sound, and can't simply run shows just to please critics.
Cable TV channels cannot live by critical acclaim alone. Looks like AMC didn't get the memo!*
*Yes, I know, nobody ever talked about AMC before Mad Men, etc.