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.