It's because the fog regularly rolls over them on the peninsula, even during the summer. So there's a lot of moisture available for redwoods, undergrowth, etc.

BTW, it doesn't apply to the foothills, nor does it apply farther south (past Cupertino or so). So Rancho San Antonio is still brown in summer, ditto Lexington Reservoir, The Dish, San Jose foothills, etc. San Lorenzo valley is green year-round, though.