Primarily the bad design and lack of modern security features (in comparison to other currencies). The most stupid decision is all notes having the same size, enabling forgers to “wash” the ink off a 1 dollar note and print e.g. a 100 dollar face on it. Whatever quality the print is, the “paper” is the right type.

Then the worldwide acceptance of the Dollar in transactions, especially in third-world countries. They don’t have means to check the veracity of such bank notes, and forgeries often stay in local circulation long before they are found.