Every round of appeals adds to the bottom-line cost: two state-level, three fed-level = $250k minimum (and it’s never the minimum.) As to the cause, it’s the system/process as it’s evolved. In my very young years in Kansas, it was the time of “In Cold Blood.” The time sequence was: crime and capture - Nov/Dec 1959; county/state trial - April 1960; executions - April 1965. So, 5 years of appeals and reviews, mostly related to the claims of temporary insanity. All things considered, nearly light speed when compared to today.
A notorious case in the Depression, the Lindbergh kidnapping/murder, resulted in the execution of Bruno Hauptmann. It was just 18 months from capture to electrocution in that case. That will never happen again but it does illustrate the change in the time factor over the years.
IOW, liberalism.