This is not really the case that the IB Diploma per se is preferred at all colleges. Some colleges are agnostic about the IB Diploma, although they may consider IB courses to be more rigorous than regular courses at the high school and may give credit, subject credit, and/or advanced placement for sufficiently high IB HL scores. The CSUs in California are examples of this type of thing – their point system based admissions does not include any points for the IB Diploma, although individual IB courses may help gain honors points in the recalculation of high school GPA used in the point system.