Because most of the "categories" Sanders tended to refer to are people who have a very real reason to expect American politicians to deliberately ignore them or else completely forget about them altogether. Calling them out by name is the bare minimum of "engagement." Pretending not to remember that they even exist is... well, not disengagement, exactly. It's just "normal."
	
	
		
		
			I don't recall a lot of this with Obama. He actually ran a fairly inclusive campaign uniting rather than dividing like Hillary.
		
		
	 
OBAMA did, sure.
But in the 8 years since he first got elected, the Democrats went on to receive an unbroken series of crushing electoral defeats. They lost state legislatures in half the country, governors, city councils, school boards. They lost control of districts and areas that were solidly democratic, they got DESTROYED in places that overwhelmingly supported Obama. This is, again, because the democratic leadership -- of which Barrack Obama has never, repeat, NEVER been a member -- spent all of their time triangulating on the least offensive stances they could possibly manage, backtracking from any statement, policy or agenda at the first suggestion by anyone that it was a "liberal" idea. They eventually got to the point that even the POSSIBILITY of being called "liberal" was something they went out of their way to avoid.
So when they were greasing themselves up to get butt-raped a few years ago, I had that very interesting conversation with a Democratic fundraiser who called me on the phone insisting that it was SOOOO important to the Country and to the State that we donate what we could to help defeat Bruce Rauner. I told him "You, Sir, will have my vote. You have my vote because the people you are running against are a bunch of psychopaths who have nothing but antipathy for people like me. You have my vote, but I am not sending you so much as a dime. I think 'I'm better than the other guy' is a good reason to vote for someone, but it is NOT a good reason to fund someone."
His reply was one I will never forget: "All of those things, those are great ideas. We can think about adding them to the platform IF we win. But with a Republican governor, none of that can be an option." And there it was, plain as day: 
Your causes might be important to us some day, but right now we just want to win.
These days, I have it on a notecard. I read it every time they call me and ask for donations: "I will happily donate to the political party that believes healthcare is a civil right, that believes that the War on Drugs is an unnecessary and unhelpful moral crusade that punishes but does not heal addicts or their families, and that a strong, well-funded education system is a society's way of investing in its own future. Those are the principals most important to me in politics, and until I see that those principals are important to YOUR party, I am not sending you a dime."