What's up with the $20 bill being sought after by counterfeiters the most when its face value is shit in comparison to $100? Stores being scammed with fake cash that a pen is needed to distinguish between genuine and fake, its a problem they encounter quite a lot. US Dollars are printed with a single layer consisting of linen and cotton, which is easy for counterfeiters to exploit (like bleaching a $5 bill imprinting $100).
That's something you don't hear about as much with Swiss Francs as they have top notch security features (they're printed vertically at different sizes). Every denomination (from 10-1000) features a globe with security ink showing dynamic color changes and a shifting arc when tilted. The bills are printed using an innovative three-layer composite material known as Durasafe which involves 8 complex printing processes.
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.