If death penalty is considered the harshest punishment in the United States, then why aren’t particularly high profile criminals placed in death penalty?

Usually if you have done a particularly serious crime, In most states you get the death penalty but I learned about the ADX Florence a while ago and it houses extremely high-profile criminals like the conspirators for 9/11 and drug lords like El Chapo or terrorists like the one of the Boston Marathon Bombers and I read most of them are serving life sentences.

So if death penalty is the harshest of punishments you can give someone then why aren't those people executed?

Edit: I was wrong about the Boston Marathon guy he is on death penalty. But most prisoners there are not so my question still stands.