The question isn’t why Dems are on the offensive; the question is why the Beltway media finds it offensive [that the Democrats are attacking the destructive policy offered by the Republicans.]The media takes the Republican narrative of destroying Medicare as serious policy, but the mere presentation of the facts by Democrats that what is presented is the destruction of Medicare is blown off as mere demagoguery or Mediscare. Why is the media so damned blind?
The answer to that question according to George Lakoff's book The Political Mind: Why you can't understand 21st-Century American Politics with an 18th-Century Brain is that the conservatives have framed the allowable possible narratives to exclude the idea that the government can possibly do anything at all without becoming a tyranny. If something does not fit into the narrative society prescribes for a given role then the individual will most often refuse to recognize the exceptions. That's because these narratives, as provided by society, are hard-coded into our brains. Those narratives are how we determine what things "mean."
People choose the roles they play from an accepted set of roles offered by society, and within each role only certain narratives are permitted. Let me emphasize - anything not fitting into that role is generally ignored unless it becomes extremely pressing. "Designing a successful government program to solve a social problem" is not an allowable narrative if your role is successful Washington reporter.
For conservatives generally the only acceptable way to get medical care is from private wealth or from current employment. That's their narrative. But it pointedly excludes the idea that the government might use reasonable insurance planning to organize the financing of health care services in advance of need and then let the private market bid to offer those services.
The answer to the question is that conservatives like the Koch brothers have worked reporters very hard to set the acceptable narratives that successful reporters are allowed to consider. There is no equivalent liberal or progressive effort being planned or conducted.
Also as Lakoff points out, merely providing the facts does not overcome the hardwired narratives built into the brain. The fantasy that such facts might change the narratives is the 18th-Century brain theory he is demonstrating is false. Frames and narratives are what matters, not facts. Those frames and narratives are hardwired into the structure of the brain. Facts that do not match the narrative will find it difficult to find a "landing place" in memory.