Bill Clinton sums up the dynamic:

"I recently heard an interesting anecdote about the 1993 budget fight. While it is probably the most progressive piece of sizable legislation to pass into law in two decades, it was a grueling fight--passing both branches of Congress by a single vote--and it still could have been better. At the signing ceremony, President Clinton found then Representative Bernie Sanders, and told Sanders that he, Sanders, should have made a much bigger public display of how he, Clinton, wasn't giving enough to liberals in the new budget. Such a public display would have provided Clinton more room to maneuver on the left."

The moral of the story is that if no one is criticizing a Democratic administration from the left, then there is no rationale or political space for that Democratic administration to operate on the left. Such criticism is thus even useful to, and desired by, a Democratic administration. If the left stays quiet, it will not be relevant.