At first blush (having just looked at their contribution system for the first time, and not having actually followed through with a contribution), I would say that open stack has built a commendably low friction contribution system. Their documentation seems clear, and the source is conspicuously located on their site. There are minor barriers, but nothing that would limit anything besides drive-by pull requests, which are of dubious value anyway.

I would tend to say that Openstack was somewhat anomalously well supported (and funded) early on. Github has enabled a lot of one-person-show projects to grow into projects with many contributors (Ansible and Cookiecutter jump immediately to my mind for whatever reason). It may be the case that larger projects that are desired by large institutions have the flexibility to build their own contribution systems that neither cede control nor introduce undue friction. It does however seem to me that Github, Bitbucket, et. al. have lowered the friction of open source projects that get actual external contributions (where the project, if not the contribution system is open source) to much smaller groups of developers, all the way down to one-person projects.

Projects that have multiple large organizations on board early on have a large incentive to implement a relatively comprehensible contribution system, as well as additional resources to bring such a system into existence. Small team or one-person projects have less incentive early on, and have less resources to implement such a system. Having Github/Bitbucket as a default for small projects means that small projects likely have much less chaotic contribution methods early on.

Since quite a few of these small projects have become strong opens source projects with many contributors, where it seems unlikely that some of them would have outside of the existence of such contribution organization options, it still seems safe to say that Github has on the whole lowered the overall level of chaos in open source contribution. It has apparently done so through its influence on small open source projects, and through it's role in raising the standards regarding the level of friction that would be OSS contributors see as reasonable, i.e. by competing with other contribution systems, it has forced other systems to lower the amount of friction in their contribution processes. So while there are still disparate contribution systems, Github has influenced things toward a lower friction state by being a strong leader in the space. And this has resulted in more coherent contribution processes on the whole despite the existence of projects that exist outside of Github.