Democratic primaries almost always end before all states have held primaries. This is nothing new.
When Bill Clinton secured the nomination in early April, he certainly didn't complain about all the voters in later states whose voice would not be heard.
A little history to put this in to perspective:
* As opposed to the general election for president which happens nationally on a single day,
the primaries stretch over time, and thus end when one candidate wins enough support to deny other candidates the nomination. In a sense, the voters in whatever states have yet to vote at that time are "denied their voice." But this is how the primaries work, every single election.
* In 2004, John Kerry sewed up the nomination in the first week of March. In 2000 Gore wrapped things up on Super Tuesday. In 1992 Bill Clinton had a lock on the nomination after the New York primary in early April.
One might argue that in all these cases millions of Democratic voters were "denied a voice." Alternatively, one could argue that the Democratic Party benefited from having the matter settled early, uniting behind a candidate, and focusing on the general election.
Obama can rightfully argue that the only way Clinton can win is if the superdelegates override the popular and delegate votes, thus "denying voice" to the voters in the primaries as a whole.
Bob Ostertag: Clinton: Stick It Out or Withdraw? - Politics on The Huffington Post