Follow the money. If they can get people who would not otherwise qualify for admission in under other protocols such as "diversity", think of all the money from Federal programs, student loans, Pell Grants, etc. that can be lapped up. Otherwise, they are stuck with people who actually qualify academically as students--the type who just might question the Party Line and be troublesome...and a lot less money.
This does beg the question of why they simply do not use race neutral means(such as affirmative action based upon socioeconomic status).