There's never enough evidence against a person to take their life away. People recieve the death penalty for ridiculous reasons, even accidental murder can land you in that spot. Accidental murder...
The reason why so many people are subjected to the death penalty is because of the cost of imprisonment. Prisoners cost money, and the government doesn't want to pay for something if they don't have to.
The death penalty costs money, too.
I find it hilarious that the needle that is used must always be STERLISIED and WASHED to prevent the spread of disease - who cares, when you're TRYING to kill people?