Probably that it's basically a weaker form of meth. I mean they're chemically similar, they have the same effects, adderall is potentially addictive, nobody wants to end up dealing with something similar to America's meth epidemic. That's not to say you can't manufacture and prescribe this drug safely, but a lot more people are going to have a lot more reservations about it than if you want to crank out a generic Tylenol or something.
I'm currently addicted to Adderall. I also have a prescription for it and I have ADHD, but I'm still absolutely addicted. Without it I would suffer, and I would seek it out through illegal means if I could not get it legally.
Maybe it's just that the cost to set up the production elsewhere is too much for the expected market. The shortage of the drugs doesn't mean there will be enough buyer to cover the cost (and make enough profit).
The other thing on top of limiting its production, it's not just for the US, it's worldwide.
Australia has a shortage of various types of ADHD medication due to this DEA production limit too.
https://www.tga.gov.au/safety/shortages/information-about-ma...
Also, the Australian Government requested increased production to cover these shortages, and the DEA rejected that request.
So those limitations have a worldwide effect due to the US being one of only a few countries that produce these drugs.