The short answer is: in 1998.
California is often perceived as a state that specializes in the making of red wines. However, that image seems to be set mainly by the wines of the Napa Valley, where Cabernet sauvignon wines are king. Indeed, the "cult" wines of the USA principally come from this region, enhancing the state's reputation, which is often the focus of wine writers.
However, the most widely planted grape variety is actually Chardonnay, not Cabernet, with 91,333 bearing acres in 2016 versus 84,584 acres, respectively (California Grape Acreage Report 2016 Crop). Furthermore, Chardonnay comprised 14.5% of the 2017 grape-harvest crush, versus 14.2% for Cabernet sauvignon (California Grape Crush Report Preliminary 2017).
Indeed, the California Department of Food and Agriculture's data show that it was not all that long ago that the white-wine grape crush exceed that of red-wine grapes. The data in the first graph are from the Crush Report for 2017, and show the crush size for each of the years from 1989 to 2017, inclusive, categorized by grape type. Eating grapes include both Table (fresh) grapes and Raisin (dried) grapes.
Obviously, before 1996 more white grapes were crushed than red, while the two crushes were almost identical in 1996 and 1997. Since then, the red crush has progressively outstripped the white. Note, also, that in 1992 the red-wine grape crush was even less than that for the edible grapes!
So, red-wine grapes have been the most important in California only for the past 20 years. This is not necessarily surprising, as the Bordeaux wine region of south-western France, globally recognized for its red wines, only moved from mostly white wines to mostly reds in the 1970s.
The most likely reason for this change in emphasis is also revealed in the same Crush Report. The following graph, taken directly from that report, shows the price paid for the various grape types during the past 10 years.
The graph makes the interesting point that the prices of Table grapes, Raisin grapes, and White-wine grapes have not changed much over the past decade. However, the price or Red-wine grapes has shown an increase of c. 50%. The has been distinctly so in the Napa Valley, of course.
The report also shows that in 2017 the Cabernet sauvignon production was up 17% above the 5-year average, while the Chardonnay production was down 13% below the 5-year average. Indeed, the Chardonnay crop was the smallest since 2011, although it remained just ahead of Cabernet as the biggest percentage of the crush (as noted above).