The cost of going for a death penalty trial is a lot more expensive to begin with.
This from a cost of death penalty website.
A Seattle University study examining the costs of the death penalty in Washington found that each death penalty case cost an average of $1 million more than a similar case where the death penalty was not sought ($3.07 million, versus $2.01 million). Defense costs were about three times as high in death penalty cases and prosecution costs were as much as four times higher than for non-death penalty cases.
Then you have lengthy appeals which cost a shitload which means the majority of prisoners on death row end up dying in prison of natural courses anyway.
You cant just lynch people these days.
https://deathpenaltyinfo.org/costs-death-penalty