I really don't think the audience for Kodi is that big. It requires you to have DRM-less, local copies of any TV or movie you want to watch, and how many people - legally - have that? Not many. I know because I'm one of those people, and use Plex, despite its faults.

Also, last I used Kodi it was one (very big) codebase. Plex is split out into server components and client apps, which is essential for Smart TV usage. You're never going to get a full instance of Kodi running on a Roku box, you'll need some kind of client that connects to a remote server holding all your videos. There's nothing stopping the Kodi team from making that Roku app, incidentally.