First of all, we must note that not all of the people in the North wanted to flee to the South. There were large numbers of refugees and something like ten times more people went from North to South than the other way around. But there were many people who wanted to stay in the North.
One reason why many wanted to flee from the North had to do with the amount of fighting that had gone on in the North. The war between the Vietminh and the French had happened mostly in the North. That meant that the North was much more devastated and there were many people who wanted to get out of the North to try to find better opportunities in the South.
The other major reason for the refugees’ movement was religious. Many Northerners were Roman Catholics. They worried that the North’s communist regime would be unfriendly to Catholics since communism is officially atheistic. For this reason, they fled to the South where they felt that the government would be more tolerant of their religion.