These aren't done more commonly in the LA area due to intense public opposition. It turns out people don't like inhaling smoke year round. Cities in general struggle to do things their citizens don't want them to do and it's not clear to me that this is a bad thing or something you can really blame the city for.
Sure, and here's what has changed between then and now:
1. A much longer and drier dry season
2. A much larger urban-wilderness interface
3. A much more organized public apparatus to combat environmental hazards (even in cases like this which require tradeoffs against more severe future risks)
The reason this is such a challenging problem today is not because everyone living in 2025 is a moron or morally corrupt and the people in the 70s were not.
