Common sense tells me it's because they either commit more crimes, or they're just not very good at not getting caught.
I would say that being raised in a low income household and living in poor, inner city neighborhoods probably has a lot to do with it. Then when you are arrested and don't have the money for a descent lawyer and have to rely on public defenders, you are far more likely to be convicted.
What I'm saying is economic status is the reason, not racism.
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Economic status isn't the whole story.
Cops tend to focus most heavily on those areas. That has statistical effects. For example, the most common drug addict is a middle class white male. The most common arrested for a drug offense is ...... you guessed it.
Now, it is obvious that the police should target "high crime areas" more carefully. BUT, this will necessarily have the effect of perpetuating its status as a "high crime are". If you're not regularly stopping middle class white males, patfrisking them, searching their car, etc, then you're not going to catch white middle class males with contraband. If you are regularly targeting black males, then that will be who you catch. Who you catch and who you do not catch creates the statistics. The chicken and the egg.....
One has to question the motivation of race-influenced policies. For example, google the numerous NYT articles documenting the fact that the NYC police targeted blacks for stop & frisks at 9 times the rate they targeted whites,
even though a greater percentage of the whites stopped were found with contraband. Were individual officers racist? Were the higher-ups who sent the officers to minority-heavy locations to conduct stop & frisks racist? I don't know specifically, but only an irrational belief can explain the decision to stick with a statistically irrational policy.
Finally, as for the "public defender" remark: These are people who go into the job because they are really pissed off about the systematic violation of rights of the citizens by police, prosecutors, and sometimes courts. These are people who should be supported by conservatives. But their ability to do their job properly is most severely hampered by conservative states, in which slashing the budget is assumed to only result in a loss of "waste". If anyone is more likely to lose with a public defender, it is because that state's legislature deliberatley crippled the public defender system by massive underfunding and laughable salaries. Public defenders care enough that they are willing to eat the laughable salary for a chance to fight government misconduct.