That is a really difficult question to answer, bpcyclist! I'll speculate below, but I'm not 100% sure how right I am on everything. Some of my speculations may seem controversial. I'm willing to be, sometimes.

* There ARE plenty of people who are NOT racist, and thankfully believe people of color should be treated the same as Caucasians, etc. Of course many (not sure of the %) of the protesters are people of color, though I know many are not.

* The ignorance about mental illness is still particularly excessive and the stigma very common in all populations. Many people see a lot of seriously mentally ill as dangerous. Obviously, many still see people of color as dangerous, when most all aren't. In the case of most of the people of color being murdered unfairly by police, there really wasn't any true legitimate provocation. For example, being killed just for jogging. Being killed even when cooperating. Being killed just for wearing a hoodie. Killed for just reaching for their driver's license. In contrast, some psychotically mentally ill do often threaten or in some way appear legitimately scary/threatening, when normally not. For example, they have a gun and are really threatening themselves, but the police fear they are threatening them. Or, they scream and move around frantically, making the cops nervous.

* Many people still demonize seriously mentally ill and look down on those with less severe illness. Many people, including with mental illness themselves, do. There's always that "Well, I'm not THAT mentally ill." Or, "People with my mental illness are different than people with THAT mental illness." Or, "They don't have a mental illness, they're just EVIL!" Yes, most people believe in the concept of evil. Though, thankfully, a lot of people in modern day now realize epileptics are not possessed by the devil, they still regard certain others as being (or at least evil).

* There is a strong history/precedent of protests against systemic racism. There isn't as much for mental illness stigma and mistreatment. The topic of racism is discussed more openly, though still not enough, than mental illness. Unfortunately, mental illness remains a slightly taboo topic, comparatively. Mentally ill, themselves, are often unable to wage a protest because of their illness, itself, and many family members are unwilling to do so. I do my best to advocate for the mentally ill, but do you think I could join some giant march on Washington D.C.? Most years I can't even join the NAMI Walk in my area to fight stigma.

* Racism, like mental illness. exists widely. We've seen that loud and clear these last few years. Racism has been talked about in the news media more than mental illness. TV shows and movies still often misrepresent mental illness, even though they are created by comparatively liberal leaning artists/producers/directors. Very often when a mentally ill person is shown in a good light, it is some movie actress or singing star (Demi Lovado, Catherine Zeta Jones) who are backed up by big money. Even the most famous writers with bipolar disorder, like Kay Redfield Jamison, often romanticize it. I admire/respect her, but she's had it much better than me, and definitely better than many others.

Most people at the highest levels in government in some countries (like mine) dare not publicly voice blatant racist remarks, but they do get away easier with ones relating to the mentally ill.

bpcyclist, you make a good point bringing this topic up. There's a good article at this link that states mentally ill are 16 times more likely to be killed by law enforcement. Another article here states that black men and boys are 2.5 (two point five) times more likely to die during an encounter with cops than white men and boys. I don't know if these stats are still accurate, but I assume they're about the same. Both are pretty sad stats!

Some may not like my criticisms or sarcasm, but let's be realistic! We're all mostly adults here.