Because they don’t control the Dems the way the hard right controls the GOP:
Looking at Obama’s actual vote in 2008 reinforces the story. According to the Edison/Mitofsky Election Day exit polls, liberals provided only 37% of Obama’s total votes. Moderates (50%) and conservatives (13%) provided far more. By contrast, conservatives provided almost three-fifths of John McCain’s votes, with moderates contributing only about one-third and liberals a negligible 5%.