A few main options, somewhat inter-related:
1) CA simply doesn't boom/grow as much as IOTL -- less people in the same space -- less congestion
2) LA, and others, don't get rid of it's urban rail networks / street cars and (somehow) keeps them commercially viable and keeps ridership up. In part this assumes a slightly less car-oriented mindset
3) Freeway construction doesn't slow/halt in the late 60s/70s. Multiple additional freeways were planned in/around LA, but Gov Brown halted some of them and affluent communities stopped some others. When they might have been reconsidered decades later, those spaces were already occupied and there are few spaces that could now be used
4) Somewhere along the way, CA develops a fast, cost-efficient inter-city rail system and avoids several costly, and embarrassing, failed attempts that discourage similar efforts in CA and elsewhere