Jared Diamond (not sure which book - perhaps Guns, Germs and Steel) makes the claim that it was because areas were productive enough that people didn't need to move and also heavily protected from other incursion from other tribes - travelling outside your groups territory or meeting a group of people you didn't know was effectively suicide. So although there was occasional trade - no people or knowledge was transferred - which explains why there's over 1000 (very different) languages.