Scott Lay, a California political analyst who maintains a well-read blog about California government and politics, says here that California’s top-two primary ballot may be responsible for lower primary turnouts. His point is that in top-two primaries, for some offices, there are a great many candidates listed. For example, in the June 2014 top-two California primary, there were 15 candidates on the ballot for Governor, and 18 on for U.S. House, 33rd district. Top-two primary ballots tend to be crowded when no incumbent is running, because candidates from all parties, and all independent candidates, appear on that ballot.
Lay also predicts that the November turnout in California this year will be poor, for the opposite reason, that there are too few choices on the ballot. His point, as far as is known, has not been made by other observers, but it rings true. Under a normal system, primary ballots and general election ballots in most states tend to have roughly equal numbers of candidates running. But in top-two systems, the two types of ballots are grossly unbalanced, with the primary ballot sometimes crowded (when no incumbent is running) and the general election confined to either one or two names.