The prosecutors are elected; prosecutors and governors think executions will gain them popularity; police are racist; juries are carefully picked out so that they make the "right" decision; defendants are frequently poor and have lawyers of the poorest quality (at least one attorney has been known to fall asleep during a trial - after which appeals were denied - while many others have failed to give vital evidence to the court or to make elementary points). The politicians pander to the basest instincts of the people.
And all for what? Does the death penalty deter? No. Does it reduce crime? No. Does it make us a more civilized society?
When people commit a crime, they rarely think of the consequences.
A nation poll (Peter Hart Research Poll) of police chiefs in 1995 found that only 1% of them thought the death penalty was an effective law enforcement tool. Asked how they would reduce crime, the police chiefs cited such measures as combatting drug abuse, building a better economy, and restricting access to guns.