In a word, closure. The family would have finality: they wouldn't have to wait for an undetermined execution date many years in the future, and have to decide if they would attend the execution.
CW could appeal a sentence of life without parole, but the appeal wouldn't take as long as the neverending appeals process in death penalty cases, where there's always someone looking for commutation or a reprieve.
Cost is another factor. Death penalty cases cost more to try because you have to do more to protect the defendant's rights, and there's extra time in jury selection and more expert witnesses. Then if you get the death penalty, there's the cost of appeals.
Colorado has only executed one person since the death penalty became legal again in 1974. Only four people on death row right now, and they are not in a hurry to execute them. He would probably die in prison before being executed, at the rate things are going.
I feel the tide is really turning against the death penalty. The Pope even came out against it the other day. Now that most states have the option of life without the possibility of parole, the number of prisoners sentenced to death has fallen dramatically.
For comparison, consider Texas, where 545 people have been put to death since 1974. Even in Texas the tide has turned. In 1999, 48 defendants were sentenced to death in Texas; in 2017 there were 4. In 1998 there were 438 people on death row in Texas; today there are 228. Here's a chart that shows the decline in the number of people sentenced to death row by year. Notice the sharp drop after 2001. That is when they implemented the possibility of life without parole.
But on the other hand, the facts of this case are particularly heinous. You've got a dead 4 year old, a dead 3 year old, and a dead pregnant wife. It could go either way.