Of course the states are free to outlaw the death penalty, that doesn't address whether or not it is Constitutional. There are plenty of things that are completely Constitutional but which are outlawed in the majority of states.

As far as popularity goes though, the death penalty has been more popular than not for all but a brief time in the 60's[0], and I think it's dangerous to conflate popularity with Constitutionality, or to suggest even implicitly that they should have any bearing on each other.

[0] https://news.gallup.com/poll/1606/death-penalty.aspx