The University of San Diego has teamed up with San Diego State to host a regional. USD's stadium is small, with 1,200 seats and it does not have any lights. Tony Gwynn stadium is larger (3,000) seats and is considered by some to be one of the best college ballparks in the nation. Like Mizzou, however, high school baseball play was scheduled at SDSU's ballpark during the times of the regionals. Although I couldn't find confirmation online, like Mizzou, it appears the conflict has been resolved.
The payoff for USD is obvious: they get to play a regional in San Diego. For SDSU, they get to showcase their stadium and, probably just as important, they get rental payments from USD. Yes, Virginia. Even cross-town rivals have reservations prices.