If I had to guess, it's because the phone market is largely dominated by Apple and Samsung, and so it'd be/has been very hard for OnePlus to actually sell their phones there.
There are minor Android players who seem to be standing their ground, so something has to be different here. Nothing, Xiaomi/Redmi, Motorola
Where I'm from OnePlus has actually sold pretty well, competing with price. But then again Finland is one of the most broke countries in EU, so lots of people are price-conscious. Hopefully Oppo will bring some replacement models with equally good price-performance ratio.
I paid 130 euros brand new for my Nord CE 5 with 8GB/128GB configuration. Couldn't be happier with the purchase. All I care is about price/performance ratio and the years of updates promised.