Most popular alternatives to Wikipedia, commercial and non-commercial, work with ownership of articles, or a stricter editorial policy. [1] This includes Scholarpedia, Citizendium, Conservapedia. It comes at the cost of volume and recency. In a sense, Wikipedia deals with all the advantages and disadvantages of modern, free democracy.
An aspect that the article seems to miss is the different demographics of Wikipedia and GitHub. (There are a lot fewer high-schoolers vandalizing GitHub repositories out of boredom.) The drawback of GitHub is not just that pull-requests must be accepted by authorized users, but also the bureaucracy of learning version control. Compared to Wikipedia, anyone with domain knowledge can hit "Edit" and add their bit.
I would rather like to see a synthesis of StackOverflow's point system, so the quality of your contributions grants you privileges for certain sections of the site, applied to wiki articles.
Another difference for comparison between GitHub/StackOverflow and Wikipedia is that you have ownership of your contributions. I occasionally go back and revise the most popular StackOverflow answers I've made for posterity. It's been years since I last made a change to one of my Wikipedia contributions. The last I remember was that someone had restructured a part of an article I'd written from scratch and added wrong information.
[1]: http://oedb.org/ilibrarian/top-7-alternatives-to-wikipedia/