The demand for pittbulls is very high (guard dogs, fighting, or otherwise), and their breeding is out of control. California alone has a massive population of rescue/shelter pittbulls who have been abandoned or abused. Other comparable sized breeds may be less likely to have such a troubled history.
Pittbulls are only the most recent headline-generating media frenzy target though. Before pittbulls it was dobermans, german shepherds, rottweilers, mastiffs, dalmatians, and even chow chows.
Another issue with headline-generating media frenzy is the misidentification of the breed. "Labrador-Shepherd-Terrier Mix bites child" won't generate the clicks.
Find the pittbull:
http://www.pbrc.net/misc/PBRC_find_the_pitbull.pdf
(hint: there's only one)