California Secretary of State Debra Bowen attributes the decline in part to the fact that there wasn’t a presidential contest last year… But she also credits counties’ housekeeping of voter registration rolls… 

“Some of it is we do a better job clearing the deadwood off the rolls…"

But she notes the state’s population is continuing to grow and the number of registered voters is not. 

In 2005 there were 16.6 million people registered to vote. As of last month, there were nearly 15.7 million. All the parties lost people but decline-to-state voters increased their overall presence—moving from nearly 18-percent of voters two years ago to almost 19-percent now