There were several stability issues related to the core design of the plugin. It didn't make for a good user experience.
In addition, Pipeline is now a core part of the user experience. It covers the cases I needed when I originally created the Multi-Branch Project Plugin. It has developer support from CloudBees and right now I don't have time to support the plugin.
Overall I thought it would be better for users to migrate their jobs to Pipeline than to continue development on the plugin or to offer it up for adoption.