Here is an excerpt from another site, ExpertLaw.com, that illustrates this:
While people who are targeted by lies may well be angry enough to file a lawsuit, there are some very good reasons why actions for defamation may not be a good idea.
The publicity that results from a defamation lawsuit can create a greater audience for the false statements than they previously enjoyed. For example, if a newspaper or news show picks up the story of the lawsuit, false accusations that were previously known to only a small number of people may suddenly become known to the entire community, nation, or even to the world. As the media is much more apt to cover a lawsuit than to cover its ultimate resolution, the net effect may be that large numbers of people hear the false allegations, but never learn how the litigation was resolved.
Another big issue is that defamation cases tend to be difficult to win, and damage awards tend to be small. As a result, it is unusual for attorneys to be willing to take defamation cases on a contingent fee basis, and the fees expended in litigating even a successful defamation action can exceed the total recovery.
Another significant concern is that, even where the statements made by the defendant are entirely false, it may not be possible for a plaintiff to prove all of the elements of defamation.
In other words, the plaintiff in a defamation action may be required to expend a considerable amount of money to bring the action, may experience significant negative publicity which repeats the false accusations, and if unsuccessful in the litigation, may cement into the public consciousness the belief that the defamatory accusations were true.