When a project is not diverse, I find it a bit too easy to blame it on the actual company who started the project, the people steering it or how inviting to contribution the project is. Though I don't want to underplay those points, I argue that there are other things that matter:
Relevance of the project: maybe the topic of your project does not interest anyone but you or your company. Sorry.
Timing of the project: you can have the coolest technology, if you are too late or too early, it will be harder to excite the crowds.
Pace of the project: the project goes too fast for others to follow or committers to accept external contributions (e.g. the pace imposed by the company's internal schedule is such that it does not allow for the external contributions to be considered by committers). Conversely The project goes too slow for anyone to be willing to bet on it.
Quality of the project: the project is running well, builds are regular, deliverables are on schedule, bugs and enhancements are dealt with quickly. The community gets what it needs, why should it care?
Amount of code: is there enough code to show the direction of the project. People are happy to work on a project but I think they feel more conformable starting from a working code base than a whiteboard.
Now if I relate that to p2, where we have a diverse community of contributors (IBM, Cloudsmith, EclipseSource, University of Lille-Artois) I think we have been lucky because p2 came at the right time, solving a real pain point. Indeed, the same year we announced p2 (EclipseCon 2007), there was at least 5 talks on how to manage Eclipse, and Cisco announced the creation of the Mayinstall project. As for code goes, we already had a functional prototype and we continued developing it in the open, holding public calls every week. All that said, I believe that p2 would still be a single-company-developed project, if the companies who joined the force had not had a business interest to contribute.
Where does leave us? Luck. I would argue that much like for any other success, the success and diversity of a project just happen to be a combination of preparation and timing with of course a zest of hard-work and persistence.