My theory is that the I-languages were much more intelligent, sophisticated, and complex than the much simpler and uglier R-languages, but that meant that it was easier for I-people to learn the R-people's languages than vice versa, so that means that eventually the R-languages just displaced the I-languages.

Essentially, R-languages were simple enough to be adopted as a lingua franca by post of Europe, whereas I-languages were too sophisticated for the R-people to adopt en masse.

So essentially, the less sophisticated languages won out because they were easier to learn.