The ancient Chinese language replaced much of the aborignal language and became part of the daily spoken speech. However, several aboriginal terms and constructs still survived and are in active use today. These two sources constitute much of today's colloquial Hoklo sublanguage. The north and the south have very different cultural and language development, thanks to the difference in geography. Northern China is flat and is easier to invade by the barbarians.
People are mobile and migrate long distances due to flood, bandits, and foreign invaders.
Whoever rules gets to decide what the official language is. The south, however, has many mountains, especially coastal Fujian. This geographical isolation makes it difficult to travel and has prevented the north from continual contact with the south. In fact, over the various dynasties, China was often divided into many warring nations. While the northern languages changed many times, the isolated south preserved many features of the ancient language, while at the same time diverged from each other due to the lack of mutual communication.