It is primarily due to the fact that prosecutors have broad discretion in what cases get brought to trial - they rarely bring cases where guilt is in question, as acquittals look bad on their record. In cases where a guilty verdict may be questionable, they will plead the person down on lesser charges or simply not bring it to trial at all.

For this reason, upwards of 94% of cases brought to trial result in convictions on some or all charges - the prosecutors simply don't press the other cases.

There is also an extensive jury selection process pre-trial, where the prosecution can exclude jurors they feel would be unable to render a fair decision after viewing all of the evidence.