There's actually one more major exception to this rule, which is Central Asia, where Indoeuropean-Iranian related people were conquered by East Asian Turkic and Mongol conquerors, and were highly mixed with (which is why those areas look fairly "Asian" today)

But this exception follows the same basic rule of having a lower population density.

The higher a population, and the more recent the invasion happens, the less settlement occurs (because it literally can't).