Earlier quoted context omitted.
As soon as you said "oat milk" is "much cheaper," I knew this had to be in Europe. It's far more expensive in the US than it is here, particularly compared to American soy and almond milk prices. I believe this will change over time due to the large increase in interest (Oatly from Sweden has a North American product and now Silk is starting their own) and the existence of a significant domestic oat industry.
Is almond and soy milk a bigger thing in the US?
According to a source, "almond (64 percent market share), soy (13 percent market share) and coconut (12 percent market share)"
http://www.mintel.com/press-centre/food-and-drink/us-non-dai...
EDIT: I should add, for those interested, that this dominance is perhaps less strange when noting that California alone produces around 80% of the world's almonds.