States like to control migration to maintain stability/the status quo.

This can be economic stability (e.g. many people move to a country to work can increase unemployment). Often states will only let people in that have skills that are in demand.

It can be cultural/social stability. Often there is a rise in xenophobia/racism when a country has a large amount of immigration (you could see this in many places in Europe). Increased xenophobia is great for the anti-immigrant and the not-so-covertly racist political parties.