I guess you're asking, why do some languages seem to change less or more slowly, compared to others. The standard story is that there must be some unknown sociological factors, but I think that's a bit facile. It is certainly true that if you have two linguistically different populations smashing into each other, you are likely to have substantial changes in language resulting. E.g. Moroccan Arabic is much further from Classical Arabic because originally Moroccans spoke Berber; Saudi dialects of Arabic tend to be the most conservative, correlated with the fact that Arabic was originally spoken only there. I don't think, though, that this really explains why Norwegian is so different from Old Norse, and Icelandic isn't.
The one social fact that does seem to have a connection to language change is urbanization. Seoul Korean has changed more from Middle Korean, compared to other especially rural dialects, and in general, linguists have found that cities are leaders of language change. Perhaps this is because cities attract speakers from many dialect areas, and what emerges is a mashing together and sanding-down of dialect differences.
