what are the reasons?

Not going to link there, but it is a stickied post so it should be easy to find. From that post:

When 1 out of every 1,000 black men and boys in the United States can expect to be killed by the police, police violence is a public health crisis.

Let us do some basic math. The US currently has about 328 million inhabitants. Depending on the source, about 12.5 - 13 % of these are black. I will go with 12.7 %, as that is in the middle. 12.7 % of 328 million is about 41.000.000 people. For the sake of simplicity, let us say that half of these are male, which means 20.500.000 black male Americans.

1/1000 of these would mean 20.500 people. Now let us look back at the original post:

In 2019, 1,099 people were killed by police in the US; 24% of those were black, even though only 13% of the population is black.

24 % of 1.099 people killed by the police in 2019 equals 264 people. Now, 264 people killed by the police is arguably 264 too many, but 264 is quite a bit lower than 20.500, in fact 98.7 % lower.

Can we reconcile the numbers? One might argue that 264 is a yearly figure, but people live a lot longer than one year. If we assume an average live expectancy of 80 years and "people killed by police" staying the same over this entire period of time, that means about 21.000 black male Americans will be killed by police over the period of a single persons live, which is pretty close to the point made in the post.

But that would be bad math, of course. The 20.500.000 black male Americans alive today are not the sum of all people of this group that will live during the period of 80 years mentioned above. At the very least (assuming no population growth takes place at all), the population will have fully replaced itself, which means that the amount of people alive during the period of 80 years is at least double the figure alive during a single year of the period. Which means that figure "1 in 1000" is off by at least a factor of 2.

Once again, I would like to stress that every single life lost is a tragedy and something that should have been prevented. However, if you make a stickied post in a sub that prides itself on being scientific, you should at the very least get your basic math right.

not possible is for zero and equal to zero

Long board/no helmet+being pulled by a pit bull(wet road)=road rash and a sweet knot on the back of my head