Why? Well, with only one exception, since 1996, the only Republicans to win statewide in California are those who did not have to win a Republican Primary prior to taking office (that is, Arnie (who won in the 2003 special election without a primary)) or those who were first in office prior to 1996 and were re-elected as an incumbent. Winning a Republican primary in an open office since 1996 (with the one exception) means you lose in the general (that is, the Republican primary pushes candidates to the right, but incumbents are immune to this).
The one exception is Steve Poizner beating Cruz Bustamonte for Insurance Commissioner in 2006. This may be because Cruz Bustamonte had the smell of "loser" around him-he was in second place in the 2003 election that gave us Governor Arnie.
Oh yeah, and Jerry Brown won Attorney General in that same election (2006)-by 18 points! (56.3% vs 38.2%)
Now, the reason for the 1996 start date for this trend is probably because in the prior election (1994), the governor at the time (Republican Pete Wilson) pushed an anti-illegal immigation initative (prop 187) to help him get re-elected in that year. It worked, but there was a backlash. Many previous non-voting citizen Latinos immediately started voting straight Democratic tickets from then on, and legal non-citizen Latinos became citizens mainly so they could do the same. I wonder if we will get a similar sized backlash in Arizona come 2012.
Net result of all of the above: Whitman and Fiorina will both lose, and lose decisively.
Source for election results: http://www.sos.ca.gov/...