Partly because Republicans have failed to tell the truth about Medicare for decades. They have enabled the Democrats to create this sacred cow because they either: (1) don’t know enough about it to articulate a different view, (2) don’t care enough about health care in general to become better informed, or (3) are so timid that they are afraid the speaking the truth will get them in trouble with the electorate.
Now their ignorance and/or timidity has come back to haunt them. Most of the elderly hold on to Medicare for dear life because there is nothing else available and they have no basis for comparison.
But the fact is that Medicare is a lousy insurance program. It would be impossible to sell a benefit structure like this on the private market.