> What are the economics or pharmacology that make fent apparently widely used in US medicine?
Let me say that I am not qualified to answer that directly, but I can point to some things that I am led to believe.
> French EMT-equivalents use it for severe out-of-hospital cases while opioid derivates are used for diagnosed, chronic needs.
First of all, I don't think EMTs are regularly administering fentanyl at the scene. I could be wrong about that.
Ketamine can keep producing (a very wide range of) primary and side effects for a good 24 hours after it's administered, whereas with Fentanyl you get a lot more control since it comes on and wears off rather quickly. I know people who have used Ketamine recreationally, and the types of intense hallucinations they report seem like they would be pretty incompatible with restraints or some forms of surgery.
Other than that, I think the U.S. seems to have deployed it more quickly because the patients and their insurers are not as cost sensitive as in single payer systems. Common forms of Ketamine are not new to anyone, whereas the most common fentanyl products are from the '90s or later. People say Americans pay a lot for health care, and this is true, but they also get most of the cool new toys, drugs, and procedures first; and so take the first-mover advantages with the first-mover disadvantages.
That said, I don't think the (local) medical uses of fentanyl are particularly important to the abuse problem. I live in Canada, and I don't think we use fentanyl medically as much, but there's a lot of it on the street. It seems to be mostly coming from independent producers rather than theft or fraud against the pharmaceutical companies that package it for use in the U.S. and Canada.