The weak voter turnout in most of the Republican presidential contests most likely reflects an enthusiasm gap for Republicans that is confirmed in recent DailyKos/Public Policy Polling survey.
The poll finds that 58% of Democrats were "very excited" about voting in this year's election, as compared to 54% of Republicans. Six months ago, enthusiasm tilted towards Republicans, 54% to 48%.
"Generally you would expect voters to get more excited as the election gets nearer. That trend is occurring on the Democratic side, but not for the GOP."