It is a combination of pitbulls being large, powerful dogs and being the fashionable choice of owners that are more interested in having a fierce animal under their control than having a balanced and happy dog. So the normal risks of a big dog are increased by neglect and poor owners.
So while there are plenty of well raised pitbulls, of the dogs on the edge of attacking people more are pitbulls because their owners raise them that way.
When I was younger, German Shepherds and Dobermans were the fashion for the same reasons.