The OP was not rejected from UIUC. They were just rejected from their major of choice. Its because the major is highly competitive and luck was not on their side.
Its not a conspiracy. The school has set slots for different student profiles and other IL students with similar profiles simply got there first. Go to the ivy forums and read who gets rejected there. It’s no different. Smart and talented is no guarantee in the most competitive programs. It’s part crap shoot.