The blaze of process arguments circulating around the Democratic Party nominating contest can tend to distract from one fairly simple dynamic that has been driving almost everything else we've seen: Hillary Clinton started this race much more popular than Bernie Sanders, then the gap between them narrowed, and now it has stayed narrow for a while but always in Clinton's favor.
There are lots of ways to slice and dice this. Sanders is more popular with white voters than with black and Latino ones. Among all racial groups he does better with younger voters than with old ones. Sanders does better in caucuses; Clinton does better in primaries. Sanders does better in open primaries; Clinton does better in closed primaries. Democrats allocate delegates proportionately, so Clinton's blowout win in Georgia netted her a lot more delegates than Sanders's narrower win in Michigan.
But here's the thing about these nuances: In the crowded, confusing Republican field there's a strong argument that different rules might have created a different result. That's just not the case in the Democrats' simple head-to-head matchup.
A great many people would like Bernie Sanders to be the Democratic nominee. But a somewhat larger number would prefer Hillary Clinton, and that's pretty consistently been the case. Sanders has said he wants to stay in the race until everyone's voted, which is fair enough. But when all the votes are counted, they're just going to show what these polls show — a lot of people like him, but more people like Clinton. That's really all there is to it.