We've known the answer for some time: mastery. It's the same reason we play sports, instruments and other "unproductive" pastimes - we desire to get good at something and enjoy demonstrating that expertise.
We all know that different ecosystems typically have different values.
But more importantly, R developers are particularly non-representative of the more general open-source ecosystem. Based on anecdotal evidence, I believe it's highly likely that academics and people who primarily identify with non-CS fields/disciplines are over-represented in the R community when compared to other communities.
(Note: At least within the text of the paper, the researchers DON'T claim that their results generalize to all OSS developers, and don't even claim their results generalize to all R developers!)
It's really quite interesting to see how it morphed over the years.