I think the problem is workflows for todo lists are incredibly diverse and trying to incorporate support for them with a single UI without cluttering it to the point where it adds more friction than it removes is an incredibly challenge.
E.g. grouping things on paper: write down a few headlines and write your stuff underneath each one of them. Want a fresh todo list daily? Write down a few lines every morning, transferring from past list as needed. Want a historical record? Well, you just got that for free anyway. And so on.
A "dumb" todo list interface solves our workflow problems by abdicating the responsibility and making us "implement" the workflow manually and just provide support for implementing workflows. And so the need to complicate the UI in ways that will almost inevitably end up adding friction goes away.
Creating a mass market application with a sufficiently "fluid" workflow support to allow this in a "smart" todo list app without adding endless configuration screens is something that'd likely be incredibly hard.
For my part, I have my own heavily customized app that works for me. But it works for me because I can tweak the code itself to my workflow whenever I decide to change any of it, and can as a result take lots of shortcuts I wouldn't have been able to with a product intended for anyone else.