Google in the process of doing a lot of revamp work on their apps and extensions. The Mountain View company has already moved a lot of their sites to a Material Design look and that process is set to continue over the course of the rest of this year. That also includes Chrome itself and that means that some extensions are going to be retired.
For a long time the company has had a Chrome Bookmark Manager plug in that brought a Material Design look and feel to the bookmarks manager in Chrome. Now that this look has moved into the built-in manager, there really isn’t a reason to have it anymore. Google, it seems, feels the same way.