I would assume it's because, unlike in 2008, there aren't three credible and well-organized candidates each with their own pool of supporters jockeying to nominate them. There's Hillary, who's getting out the Dem faithfuls, and Bernie, who's getting the young and the independents but doesn't seem to have a super-strong ground game. The Republicans have 6 "big name" nominees, all of whom have sold some kind of message about how they have a credible chance to their supporters, thus enthusing them to get out and vote. Once the actual election rolls around, and Rubio or Trump are talking about building border walls and cutting welfare, the Dems will be scared enough to turn out, especially if Hillary uses the same GOTV shit that Obama used back in '08 (and why wouldn't she?).