This is not going to be an explanation per se, more of an insight really.
The crime rate in black neighbourhoods is statistically significantly higher than in non-black neighbourhood. This trend can also be observed i latino neighbourhoods btw. Many young black men (women too, but men dominate this. We must fight for equality!) get arrested even before adulthood, some for smaller offenses like possesion of pot, some for bigger infractions, like violent crimes, armed thefts and so on. Statistics also show that a significant portion of those incarcerated do commit more crime after being released, and so when in court again, their previous offenses count against them and contribute to a higher sentence. That is a standard policy regarding repeating offenders regardless of their skin color, but unfortunately black people are, statistically speakinh, significantly more likely to break the law again.
Obviously, some racism can be and is present in the system, but it isn't institutionalised, it's racist and bigoted individuals in positions of power.
The crime rate in black communities can be tracked down to several reasons, some historical, some less so. One of the more significant ones is that the percentage of single mothers in black communities has been steadily and rapidly increasing in the past few decades, That results in lower household income and in pressuring the children (teens, young adults, whatever you wnat to call it) to seek income, and because kids are generally speaking stupid and impressionable, molded by their environment, they seek the 'easy money' of criminal activity, as it seems to be one of the default options.
All of this is a gross oversimplification, the problem is way more complex than this, but I hope it can give you some insight.