UIUC doesn’t release acceptance rate for most majors, just school specific acceptance rates (and CS): Admit Rates, Undergraduate Admissions, University of Illinois Urbana-Champaign
UIUC majors will also have higher acceptance rates for in-state vs OOS students.
Beyond volume and strength of applicants being part of acceptance rates, so is the number of students that are allowed to be in the program. Many schools cap the size of the CS major because of the difficulty in hiring CS Profs/adjuncts. Other majors can be capped too, commonly some engineering majors, business majors, and nursing majors are.