To ensure that immigrants convince themselves that it is beneficial to become British citizens. Anything which has no effective barrier to entry has no effective value. This is not true of British citizenship which has huge value.
We are told that it is beneficial for a student to pay £27000 in student fees to become a graduate.
I suggest that a very large fee be introduced in order to become a British citizen. Payable up front would be a good idea, too.
There is a large benefit in becoming a graduate, but there is probably a larger benefit in becoming a British citizen.