Because the judical system of every country in the world includes human judgment to a greater or lesser degree and humans are easily influenced. An innocent person may be found guilty on the basis of race, religion, gender, the severity of the crimes accused, personal biases on the parts of the triars or triar of fact, and/or very bad coincidinces.
An innocent person may also be found guilty if the prosecuting party bribes or otherwise influences the triar of the fact, or if the defending party is so incompetent as to incriminate its own client. '
Furthermore, regardless of the country, regardless of the time period, regardless of the judicial system, trials (or other forms of guilt-determination) have always been a game, and, while it is theoretically easier for an innocent man to win than a guilty one, guilty men have won and innocent men have lost.