The people who care about executions being humane are generally opposed to the death penalty. People who support the death penalty generally want suffering to be inherent to the process. Only limit is whatever the Supreme Court deems “unusual”. Cruelty is allowed by the Constitution as long as it is “usual” cruelty.

In states that have death penalty (and federal when we have a president who supports death penalty), it’s the pro-death penalty groups - the ones that want it to cause suffering - that get to pick the process.