The difficulty here is the collision between explicit discrimination by the state (prohibited in this case by the CA constitution) and de facto discrimination by private actors (see the significant under-representation of many different demographic groups in positions of power and wealth). CA sought to use the power of the state to legislate some private behavior to reduce the negative impact of de facto discriminati…
All that’s to say that if shareholders (who theoretically appoint and are represented by the board) as a group have legitimate and real preference differences from the average person from the these other minority groups, you can get easily get boards that are unrepresentative of the minority groups.
Now, as to why those shareholders might have preferences that differ from those minority groups so much, you can start making class based or structural inequality/racism arguments. But I’ve written enough :)