Why is there no rain during the summer in California?

Just taking a guess, rain happens when air which has a certain humidity level cools down. The lower the temperature, the less water vapor the air can support, and at some temperature point the water vapor comes out of the air as rain or snow. This usually occurs when colder air blows into the area and mixes with hot air.

California is probably so warm during the summer that there is no cool air in the vicinity, or at least air cool enough to bring the warm air down to the point where it rains. Not very often, anyway.