Turnout was only supposedly higher in the 2016 race because the race wasn't already clinched halfway through the primary process, thus big states like New York still had competitive races—not because of any greater enthusiasm.
@jmyrlefuller New York was never competitive...the turnout there was for native son Trump, but you may have a point about the length of the 2016 primary season.
I believe the comparison Breitbart makes is between 2008 and 2016 and one difference is that in 2008 California moved their primary up to sometime in early February while in 2016 it was in June, well after Trump had clinched. Over a million more voters participated in the 2008 primary than in 2016. Other states may also have changed their dates, but you get the idea.
So it may not be entirely fair to say the turnout was greater in 2016 because the race was competitive longer, when there are anomalies like California involved which add at least a million extra votes to the 2008 turnout total.