I think that was because they could more effectively market their "pain is bad mkay" strategy in the culturally and politically more homogeneous market that is the US.
The European healthcare market is completely different for each country in multiple dimensions. Be it political, the way insurance is structured, laws are setup and governmental agencies handle them. That means while it is entirely possible to sell the pills in Europe they won't be prescribed in the amounts necessary to jump start the "vibrant free market" for them the US has. So they are just an opioid that is prescribed in extreme cases.
I dont think the idea that patient pain is bad was unique to perdue and their drugs. Many European countries have private health insurance, with reimbursement quite similar to the US.
Like I said above, I think it would be extremely shallow thinking to claim that there is a single reason.
If I were to pick a leading difference, I would say that the US has embraced trained consumerism to a greater degree than most European countries. As such, the idea that a simple pill/product will make a problem go away has more traction, both with prescribers, patients, and abusers.
You see this difference manifest in many cultural and social forms, where people in the US are especially prone to "quick fix" marketing and products that offer escape and excitement through consumption.
This is one thing that leads into higher rates of substance abuse in the US than Europe. For example, the US has a higher rate of alcohol use disorder than most European countries, despite most of the countries having more permissive laws around alcohol and more consumption of it on average.
