Kentucky was a closed primary: Bernie Sanders has consistently seen almost a third of his support come from voters who identify themselves as independents. In West Virginia, exit polls reported by CNN showed that Sanders lost among Democrats but won among independents — a group that made up a third of the electorate. (It was apparently the fifth state in which independents gave Sanders a win.)
With the exception of the unusual, multi-day Democrats Abroad primary, Clinton has won every single contest that was a closed primary — meaning only Democrats could vote and it wasn't a caucus. (Sanders has won far more caucuses than Clinton.) The fact that only Democrats could vote in Kentucky played to Clinton's advantage. Clinton has won with Democrats in every state for which we have exit poll data, save three. Sanders has won with independents by the same ratio.
There are more black voters in Kentucky than in West Virginia: The density of the black population in a state has been an excellent predictor of the results of the contest. There are a number of wide divisions in the Democratic electorate — young people vastly prefer Sanders and old people prefer Clinton, for example. But those demographic groups are pretty consistent across states. It's when a state has a large black population — a group of voters who have consistently strongly backed Clinton over the course of the primaries — that Clinton is much more likely to win.
Again, there are no exit polls from Kentucky. But in 2008, black voters were 9 percent of the electorate in Kentucky to 3 percent in West Virginia. (Last week, the percentage in West Virginia was the same.)
Clinton spent money: As Clinton has tried to turn her attention to the general election, she has consistently been outspent by Sanders in recent contests. That wasn't the case in Kentucky — at least in terms of TV ad spending.
The protest vote appears to have been smaller: After Sanders's victory last week, we noted that the number of people who voted for neither candidate was higher than in any other state so far.
As of writing, more than 5 percent of Kentucky voters picked "uncommitted" as their choice for president — a built-in protest mechanism. Last week, about 1 in 8 votes went for a non-Clinton, non-Sanders candidate. (The main beneficiary? A Huntington lawyer.) This is probably related to the fact that fewer independents turned out, but in a contest between an insider and an outsider, fewer protest votes should help the insider.
There's only one way in which a Clinton win would actually make a difference, of course. Since the Democrats allocate delegates proportionally, both candidates will get about the same number of delegates — good news for Clinton and bad news for Sanders, who needs to start winning delegates by overwhelming margins.
But Sanders has argued that a late sweep of states would demonstrate to superdelegates that the will of the party is with him. That argument depended heavily on continuing to win state contests, however little difference it made for his delegate total. He hoped to make the argument that he had momentum — an argument he made coming into the New York primary as well.
What happened in Kentucky had nothing to do with momentum one way or the other, as we articulate above. It was a state that offered Clinton fundamental advantages, allowing her to squeak past Sanders by the skin of her teeth.