Unlike many countries they've never demonetised any notes, even silver certificates from the 1800s are still legal tender (although genuine ones have collector value, so you'd be an idiot to spend them). So even if they introduce security features people can just forge the old version.

In 2013 the $100 bill got a security feature upgrade. The $20 did not.

Is this our various currency questions sock puppet? Asking insane questions or incredibly boring ones they answer themselves?

$20 dollar bills are the most common denomination in retail, and ATMs here exclusively hand out 20s. Even with prices going through the roof, you hardly ever see higher denomination bills out in the wild. A shop owner may look at you funny if you hand over a $100 to pay for something (and would definitely get the pen out), but would probably not blink twice when handed five $20s, even though counterfitters target them just as much. (Also note that we no longer make any bills over $100 at all, so your only choices over $20 are $50 and $100...)

We do have some currency modernization programs underway, but they will not effect the size of the bills. We seem to think it would be a disaster if our currency were like every other country's currency. We need to think we're Exceptional, I guess.

I believe something to full with how USD of note widely accepted across multiple boarders/countries. It's been the standard for a long time when trading oil iirc too.

Sounds like US currency is easier to fake, which is another reason it's sought after by counterfeiters. Why try to replicate something like Swiss Francs that's less accepted and harder to replicate?

It seems like you and others mostly answered your own question, but another aspect is that often paying with $100 bills raises an eyebrow and comes under more scrutiny - after all, the shop is out $100 if they accept a bad one. People are much more likely to carry and pay with $20 bills, so it's significantly (I would guess more than 5x) easier to pass a few bad ones off here and there. I think gas stations used to be a common way to work counterfeit bills into circulation, as are retail shops, both of which are going to be less diligent than banks.

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.

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.