It really is odd that it seems to correlate so well with longitude. I can't think of anything that would actually be a causal link there

I think you're on to something with wealth playing a factor. It would explain Greece and Portugal being outliers in their regions, at least

I'm curious to know why Denmark is so unusual. Normally the Nordic countries see pretty similar results on this sort of thing, but Denmark compared to Norway or Sweden is a wider gap than almost any other neighbouring pair