Ideology of conservatism.
I hope this helps!
If the border with Laos had been closed and North Vietnam had not been able to operate in the country, the communist side would not have been able to stock up on resources, men, ammunition, weapons, food and medicine through the HoChi Minh Trail.
Therefore, the forces of the North that were fighting in South Vietnam would have been out of supply, and their defeat would have been imminent.
On the mayflower they had to follow the mayflower compact, if that helps ?