Answer:
Taxes
Step-by-step explanation:
The citizens of a state pay taxes to support the education of citizens of their state which rebounds to the welfare of their state. They are given a reduced tuition rate because the state wants to be certain that the citizens of that state are well-educated. Out of state students have chosen not to support their state, but to attend out of state and take advantage of the base state and its resources. The state of the school must attempt to recoup the costs that in-state residents must pay, rather than paying for the education of those out-of-state students.
Rather than a “burden” on out of state students, the view is that in-state students receive a “discount” because they (and their families) have been paying taxes to that state, perhaps for decades. Politically, voters of that state fund state universities because — in part — they know they get a discount if they or their family members attend.
Perhaps the question in response to your question is: What’s wrong with just attending your own state’s public university system, since it will be less expensive?
State university systems charge less to their own taxpayers than they do for non-residents. That made a great deal of sense when the California system was free to residents. Why should you subsidize the education of those who aren’t your own citizens? That was the argument.
The problem now is that states charging much higher tuition to non-residents see that as a lucrative revenue stream to subsidize their systems and starve the schools. That is not particularly sustainable and it’s one of the reasons education overall is just more expensive than it needs to be.