It is true...statistics show that there are more crimes committed by black than whites.
However...there are some misconceptions with these statistics. For example the type of crimes committed. White collar crime, things that happen in the business word are more often committed by white people than black people. Black people are often involved in petty theft, robbery, etc. As far as crimes such as murder etc...those are a bit skewed but there are just as many white, as their are blacks committing those crimes.
I took a criminology class and there are whole sections devoted to physical appearance, biological factors, race, environment, in relation to criminal behavior. So to answer your question on why...there are many answers.
There are several theories and criminal classifications which hone a person based on the crime committed and/or the factors that contributed to the criminal behavior.
Environment plays a big role....there are statistics that show that those that come from a poor class, broken type neighborhood often have a higher crime rate.
Others theories are role models..such their peers and their parents. People are most likely to follow their footsteps.
Other theories are purely biological...some criminologist believe it has to do with genes and those possessing certain physical attributes are a sign that they are more likely to commit crimes. Some physical attributes associated with higher criminal behavior are protruding jaw line/lips, big nose, big ears that stick out, a wide forehead etc. And those with black skin are known to have a big nose, big ears etc.
Other theories...are things like those that have tattoos, piercings are more likely to commit crimes, because in early days it showed how the act of getting a tattoo shows that they are often insensitive to pain...another sign of criminal behavior.
However, I believe the overall contributing factor is the environment, the type of environment you are in...and it holds true for all races. For if you put a white kid in a black neighborhood that has a high crime rate and raise him, he will just as likely have criminal tendencies as another other kid on the street.
I could go on and on...but there is no one true answer...many theories involved and often these theories interlace with each other.
What many also dont realise is that if an area is largely populated by a particular race, the crime stastistics by that particular race will obviously be higher.