Haas has an overall acceptance rate of 18%. That is lower than all schools but Harvard, Stanford and possibly Columbia (which is tough to gage because of their ED round). Even Wharton is 20% and others go up from there.
I believe the issue is that Haas is really small, and serves a really popular area. For example, New York has about 8 million people (I think) and is served by Columbia (about 750 full-time and 16 month program), NYU (375) and has substantial location advantages in attracting students from Wharton (800), Harvard (900), Cornell (280), Tuck (250) and even Yale, Darden and Fuqua.
Chicago has fewer people but is served by Chicago (550) and Kellogg (525) and has advantages with Michigan (440).
The SF Bay Area has about 6 million people and is served by Stanford (375) and Haas (250). Together, that's not much more than many of the mid-sized schools. There is also the huge population of Los Angeles nearby with just the additional seats of UCLA (325) to cover the entire region. In fact, in addition to all the hopefuls from California, hopefuls for a thousand miles in any direction do not have another elite business school to choose from. I think it's simply that there are just so damn many people in the region and hoping to move to the region competing for so few seats at the three elite/ultra-elite schools.