A currency’s value reflects confidence in the economy issuing it. The dollar is backed implicitly by the size, productivity, and stability of the US economy.
Lets say you're Germany, US imports make up only 6.9% of total import value(Netherlands is 7.1% and China 12.5% on top) closely followed by Poland with 5.9%.
Take most countries in Europe, US imports are a rather small very modest portion.
So it doesn't make that much sense that US dollar would be particularly valuable.
After all - you really don't actually even buy that much stuff from US, thus the need for the US dollar isn't particularly large, and why would it be?
On the other hand, if you want to buy oil however, you need the dollar. That is what makes the dollar valuable.
Now if you can buy oil from Iran, Russia or whatever in some other currencies, why would you need the stinking dollar?
I mean, you do import some US goods, but not that much really.
Why would you have confidence in an economy that regularly has insane trade deficit (1 trillion now)?
Once the petrodollar goes, thing will go tits up. And it will, because there's only 60years of oil left in the global oil reserves in the tank.