Clinton’s winning because more Democrats want her to be the nominee.
That's the conclusion of the analytical report at 538.com co-authored by Nate Silver. (h/t Jerry Stoops)
... turnout was much higher in the Washington primary than in the caucuses, and Clinton did much better. Something similar happened in Nebraska, where Clinton lost the early March caucuses by 14 percentage points and won the early May primary, in which no delegates were awarded, by 7 points.
Nebraska and Washington are part of a pattern. As Sanders fans claim that the Democratic primary system is rigged against their candidate and that Sanders wins when turnout is higher, they fail to point out that Sanders has benefited tremendously from low-turnout caucuses. Indeed, if all the caucuses were primaries, Clinton would be winning the Democratic nomination by an even wider margin than she is now.
... [Lots of data analyses here.]
What would happen if the primary system conformed to each candidate’s best-case scenario? (All closed primaries for Clinton and all caucuses open to independent voters for Sanders.) If every state held a closed primary, Clinton would beat Sanders by 19 percentage points and have a 654 elected delegate advantage, we estimate. If, however, each state held an open caucus, Sanders would beat Clinton by 22 percentage points nationwide and have a 496 elected delegate lead. Of course, neither of those scenarios would happen.
Realistically, if you throw everything together, the math suggests that Sanders doesn’t have much to complain about. If the Democratic nomination were open to as many Democrats as possible — through closed primaries — Clinton would be dominating Sanders. And if the nomination were open to as many voters as possible — through open primaries — she’d still be winning.
So any way you cut it, Clinton's numbers are higher than Sanders' numbers. The continued "discussions", shall we say, about the composition of DNC committees is another matter.