From the article:
It is hard to remember a time when the polling and expectations were so universally different from what really happened. At the same time, It is worth remembering that polling was generally on target for the Republican race. John McCain won, as expected, by splitting the Republican vote with Romney and winning big among Independents. Independents accounted for 37% of the Republican Primary voters, a bit higher than projected.
So what happened with the polling on the Democratic race? There are several possibilities.