There are many many different reasons why the UK voted for Brexit. Each person will have a different story on why. however, having your borders defined and controlled by people you can elect is very important, as are most of the laws that define our country and society.
We can decide for ourselves, what our immigration policy should be, and it will be in our interest.
Being forced to have an open door policy to every tom, dick and harry by the EU law was and is an issue and contributed to having to leave. 3.7 million EU citizens are now in the UK, and 2million of them came in the last 18 odd years !!!!. That is alot of people to absorb and has distorted many of the things we took for granted. As a proportion of our overall population it is quite significant, and has had a significant effect on the UK and therefore what people view of immigation.
Having close to 2million EU citizens migrate to the UK in the last 18 years has caused a lot of issues over jobs, public services etc... we need to turn off the tap for both EU and UK citizens and use the time to recover and rebuild....
We have had also very high non-EU migration, and the new immigration policy will look to bring those numbers down aswell. So, overall, less opportunity for migration, not more.. despite what some people on this forum think!
https://migrationobservatory.ox.ac.uk/resources/briefings/eu-migration-to-and-from-the-uk/