It's not exactly rocket surgery, is it? Because they are fighting on the other side of the world. I feel like the article glosses over this way too quickly.
It's not surprising the US aren't winning. It's surprising they are at all able to do something that resembles waging war at that distance. Wars are incredibly hard to fight away from home. On the other side of the world? There is basically only one country in the history of ever that has been able to do that.
The article does go on to discuss the Kuwait case and that's a meaningful discussion to have, because actually winning a war so far away is surprising. It's interesting to dig into why that might have been, and the article briefly does. But why they aren't winning is less interesting.
For more meat on how important a factor distance is, Bueno de Mesquita's The War Trap presents one of the most accurate models for predicting wars[1] I know of. That model measures the fighting ability of a country by two factors:
1. It's at-home military power as a sum of population size, metal production, fuel consumption, military expenditure, etc.
2. It's power projection as the distance between its capital and the closest point of enemy land.
Note that the second number is weighted equally to the sum of all other factors, because wars are a logistical nightmare.
[1]: https://entropicthoughts.com/war-what-is-it-good-for