Because the net difference between dialects are the sum of all differences from 2000 years ago to now, while 2000 years ago, less time was elapsed, the net diff

Because the net difference between dialects are the sum of all differences from 2000 years ago to now, while 2000 years ago, less time was elapsed, the net difference was much smaller. The net difference today is the net difference 2000 years ago plus all the new variations since then.