I've said it before, and I'll say it again. It's always advisable to take every poll with a grain of salt.
The Elon University poll, as it turned out, is an outlier, i.e., a statistical observation that is markedly different in value from the others of the sample, as defined by Merriam-Webster. (If it's any comfort, Elon also lowballed Donald Trump in the same poll on the Republican side. For some odd reason, this poll uncovered more undecided voters.)
Source: Real Clear Politics
North Carolina is an odd case, given that up until March 7th -- less than a week ago -- the polling was showing Sanders cutting Clinton's lead from 17 points down to 10. All that's happened since then is that Sanders has pulled off "one of the greatest upsets in modern political history"; (i.e., Michigan; see below)
In the end, the NC Dem primary wasn't the rout anticipated in the polling, but Clinton nonetheless ended up with a comfortable margin of victory.
Sources: Real Clear Politics, CNN 2016 Election Center
Why The Polls Missed Bernie Sanders’s Michigan Upset. (FiveThirtyEight, 3/9/2016)
With a polling miss this big, no single factor is likely to explain it, so more than one answer could be correct.The first reason listed: Pollsters underestimated youth turnout.
What I find most interesting, however, is that 7 daily FOX 2/Mitchell polls in 6 weeks (Jan. 25, Feb. 4, Feb. 15, Feb. 23, Mar. 1, Mar. 6, and Mar. 7) consistently and significantly overestimated the spread between Clinton and Sanders.
Dear Seth, Wishing doesn't make it so. The Florida WTSP poll was an outlier. (3/15/2016)
While you were sleeping, GOP Presidential candidates pick up a few delegates. (3/13/2016)
Lone Idaho poll misses Cruz express. (3/9/3016)
Trump, Clinton win in Mississippi; Rubio in free fall. (3/9/2016)
Trump wins as expected in Michigan, Sanders scores upset. (3/9/2016)
Trump, Rubio underperform, Cruz well exceeds expectations in GOP Louisiana primary. (3/6/2016)
Trump squeaks past Cruz in Kentucky GOP caucuses. (3/6/2016)
It's a Ted Cruz caucus kinda day: First Kansas, now Maine. (3/5/2016)
Pollsters didn't have a clue that Cruz would win big in Kansas caucus. (3/5/2016)
In a Super Tuesday smackdown on Sarah Palin, Alaska voters give Ted Cruz a win in GOP caucus. (3/2/2016)
Cruz does what he needs to do in Texas, exceeding Super Tuesday expectations. (3/2/2016)
Super Tuesday in Minnesota: Apparently, there was little to no interest in polling voters. (3/2/2016)
Cruz gives Trump a Super Tuesday scare in Arkansas. (3/2/2016)
Super Tuesday brings expected Trump, Clinton wins in Alabama. (3/2/2016)
No Super Tuesday surprises in Georgia as Trump, Clinton win easily. (3/2/2016)
It's definitely a Super Tuesday for Bernie Sanders in Vermont. (3/2/2016)
As expected, Trump and Clinton win in Tennessee on Super Tuesday. (3/1/2016)
Trump wins big in Massachusetts GOP primary, but overall he gets smoked by Clinton and Sanders. (3/1/2016)
Cruz surprises in Oklahoma GOP primary; Sanders wins Democratic race. (3/1/2016)
Clinton, Trump win in Virginia; Rubio finishes a strong 2nd in GOP race. (3/1/2016)