It's about delegates and the nomination is not decided based on who won the most states or who came in first in the biggest state. Sanders has to win big enough here to offset Biden's totals in southern states. Sanders did best in the rural north last time, so he could come in first simply by repeating that again. Biden could offset that by doing better in Southern California which favored Clinton in 2008 and 2016. If Sanders is doing poorly in Southern California, then coming in first statewide isn't going to mean very much in terms of delegates.
I don't think the 2008/2016 comparison is useful. With his strength among Hispanics and younger voters I would be very surprised if SoCal isn't one of Sanders' best regions.
