Language is a uniquely human skill. That's why studying how people learn and use language is crucial to understanding what it means to be human. Given that most people in the world—an estimated 60%—are multilingual, meaning that they know and use more than one language, a researcher who aims to understand language must also grasp how individuals acquire and use multiple languages.
I think the glaring mismatch here on the global north/south line is likely due to the fact that economic “development” (that is participation in global capitalist systems) creates the kind of university systems and scholars that have the means and connections to publish in high-impact journals while also having a negative effect on linguistic diversity in the country as a whole due to economic incentives for linguistic assimilation among other things.
It’s interesting to look for countries on their map that are more or less even on the research/impact linguistic diversity scale. One is China, although it seems to me they are rapidly heading away from linguistic diversity in terms of language policy while also dramatically increasing research output. Canada also is pretty even, I think that is in part a reflection of a relatively high Indigenous population proportion (~5%), as well as having a total of 217 languages recorded in 2016 (compared to somewhere between 350 and 400 in the US) despite having about a tenth of the US population.
(Numbers from Wikipedia and Canada.ca language statistics, on mobile too lazy to link. Recent numbers for the number of languages spoken in the US are surprisingly hard to find, latest I found with a quick search was ~430 in Grimes 2000 and 381 from the 2011 Census)