There are many factors:
The first factor is the legacy of history. In the early years, the North was wealthy, so it looked down on Southerners. Later, as the South developed, it began to look down on Northerners. Feng shui takes turns, and so does the mouth.
Another factor is that when northerners make mistakes, they make a big deal out of it, while when southerners make mistakes, they sometimes accept them without naming them or even criticizing them. This is a serious double standard in the guidance of public opinion.
Another factor is that the north was the industrial base of New China, but after the reform and opening up, a large number of people were lost. At the same time, the economic growth brought about by industry was slow, resulting in many remaining people with low quality.