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.
I'm guessing that, among other factors, a cashier is going to scrutinize a $100 much more than a $20. So if you forge $20 you have a better chance of getting away with it.
Another thing is the "legal tender" thing. In USA, unlike most other countries, the design of the bills hasn't changed for a very long time, and shops are obligated to honor even very old bills. This means new security measures are ineffective, because forgers can just forge old bills without those security measures and it wouldn't be suspicious.