It's because he hasn't developed an expertise on the subject and maybe he really does believe it will be "that simple" to change the system completely. The Krugman article Oxy directed me to this morning really does explain it all. 

If we really think about this, what is it about the Sanders plan that people are attracted? I think they really do believe it will be somewhat free to them, and can be achieved just by raising taxes on the wealthy and raising corporate taxes. I wonder about that kind of selfish warped thinking? Healthcare is a shared responsibility, no one gets out of paying for it. Individual taxes will increase and it will be a dramatic increase. I wonder how one does convince the American public to do that. Middle class taxes will rise the most, and if we don't begin to regulate other aspects of the system first, those taxes are going to be incredibly steep. There is absolutely no way it will happen, Republicans will have the upper hand in the tax debate on that one, and the middle class will still be made to feel that they are getting screwed while everyone else is getting some benefit. 

We can't start over. That is an insane notion and is an indication that Mr. Sanders and his supporters don't really understand the nature of the large organizations. .

I also find it interesting that Sanders would rather make a statement with his healthcare legislation that he has introduced yet again, than to have actual impact with legislation i.e. regulation of hospital costs, etc. I understand he is introducing his Medicare for all Legislation for the 20th time. That sounds an awful lot like Republicans who continue to re-introduce legislation repealing PPACA, knowing it can never happen. In that he also has recently decided the program cannot be run by their respective states and moved on the a system run by the feds further indicates he does not have a real grasp of what it is going to take to continue our move to get everyone access to healthcare.