check out the bolded sentence. pennsylvania and ohio were "closed" primaries meaning unless independents and republicans re-registered with the democratic party, they didn't vote.
here are the 3 reasons:
a) Hillary Rodham Clinton is such a polarizing figure that everyone who ever considered voting Republican in November, and even many who never did, will go to the polls to vote against her, thus jeopardizing Democrats down the ticket – i.e., themselves, or, for party leaders, the sizeable majorities they hope to gain in the House and the Senate in November.
(b) To take the nomination away from Obama when he is leading in the elected delegate count would deeply alienate the black base of the Democratic Party, and, in the words of one leading Democrat, “The superdelegates are not going to switch their votes and jeopardize the future of the Democratic Party for generations.” Such a move, he said, would also disillusion the new, mostly young, voters who have entered into politics for the first time because of Obama, and lose the votes of independents who could make the critical difference in November.
(c) Because the black vote can make the decisive difference in numerous congressional districts, discarding Obama could cost the Democrats numerous seats.
One Democratic leader told me, “If we overrule the elected delegates there would be mayhem.” Hillary Rodham Clinton’s claim that she has, or will have, won the popular vote does not impress them – both because of her dubious math and because, as another key Democrat says firmly, “The rules are that it’s the delegates, period.” (These views are closely aligned with Speaker Nancy Pelosi’s statement earlier this year that the superdelegates should not overrule the votes of the elected delegates.)
Furthermore, the congressional Democratic leaders don’t draw the same conclusion from Pennsylvania and also earlier contests that many observers think they do: that Obama’s candidacy is fatally flawed because he has as yet been largely unable to win the votes of working class whites. They point out something that has been largely overlooked in all the talk – the Ohio and Pennsylvania primaries were closed primaries, and, one key congressional Democrat says, “Yes, he doesn’t do really well with a big part of the Democratic base, but she doesn’t do well with independents, who will be critical to success in November.”