It's a matter of the lengthy court process through our justice system. Appeallate courts and the State & Supreme Courts have many applicants who want their cases heard. It takes a lot of time to sift through the small fraction that actually are considered by a higher court.

To give you an example of how long the process can take, Ted Bundy was captured after escaping prison (for the 2nd time in one year) in February 1978. During the time he was free, he killed or assaulted 6 Florida women and killed a little girl. He was convicted and sentenced to death by two separate juries in 1979 and 1980. Due to the appeals process (Bundy even petitioned the U.S. Supreme Court), Bundy wasn't executed until January 1989.

In Ohio, when the death penalty was reinstated, there was an inmate who made headlines in 1998 because he waived all of his appeals. He would have died sooner had Ohio been using the death penalty prior to 1998. (I can't recall how long capital punishment was inactivated prior to 1998 here.)

Remember before Tim McVeigh died - there was all that talk that he could escape the death penalty due to a technicality involving something the FBI did wrong.