Aside from any campaign trail decorum (which Trump threw out the window a long time ago), this reluctance also seems to be the product of a cultural shift in which it's somehow viewed as a worse offense to call someone a racist than to actually BE a racist. Jelani Cobb had a great take on this a couple of years ago, discussing how bigots were suddenly the ones claiming they were oppressed because people called them out. He wrote, "The prevalence of specifically racial offenses shouldn't blind us to a kind of bigotry jiu-jitsu that often follows in the wake of these incidents, in which the offenders quickly flip the script to portray themselves as the victims of intolerance toward their own intolerant remarks."
Do I think that Sanders and Clinton agree that it's worse to call someone a racist than to be one? No. But I think their hesitance is a sign of how much this attitude has seeped into our culture. They made a calculation, and decided there would be a cost to them to confront his racism head on.
As Trump racked up more primary victories this week, there is increased urgency behind exposing who Donald Trump really is and the practical and psychological damage electing someone with such blatant disregard for the rights of people of color (and women and lots of other people) would do. Some people argue that Trump doesn't have a coherent ideology and just spouts the first thing in his head or twists his positions to reflect what he thinks people want to hear. His record is certainly scattered, but as John Oliver said in his brilliant Drumpf takedown, "you are either racist or you are pretending to be, and at some point, there is no difference there."
So we can't stop talking about the danger of electing a racist like Trump to office. And chances are, if you're a person of color, you're already doing that. Some white people are too. But as comedian W. Kamau Bell pointed out,
Whenever Ben Carson says batshit crazy nonsense, Black people rise up, and let him know that he needs to STFU. Whenever Raven-Symone pops off, we put her cap back on. We even handled Rachel Dolezal for you. Yes, we also make jokes and come up with clever memes and hashtags, but at the core of all that is that we are letting these people know that they are embarrassing us as Black people. It is time, white people, for you to finally step up and recognize that you also (even more so) have a responsibility to your race. It is up to you to silence Donald Trump. Don't just insult him and make fun of him. You have to connect it to your race. Recognize that he is embarrassing you as a white person.
Whether Sanders and Clinton are ready to join us, it's well past time for white people to "come get our boy."