GitHub got off the ground on-selling an emerging open-source product as a service with some innovations around data visualizations / analytics and community structure, customized for the web. Their growth really comes down to the right set of features at the right time, a low friction setup, good price point and good flow of communication with their user base.
The perceived stagnation is likely a side-effect of scaling the operation to fit with increased demand and the growth and expectations of their private & enterprise (paying) customers, who have become notably more high profile as the years roll on. With it comes the difficulty and expense of providing a dependent, secure infrastructure and a more refined and audited code base to fit the needs.
It's a diverse community here and while some groups consistently demand feature freeze (hating on 'bloat', 'features coming from marketing', focus on 'core product'), others are only convinced that a products relevancy is based only on cutting edge features ('we need feature a, because b', 'product c is irrelevant because product d offers a'). To offer refinement that appeals to both camps is a delicate tightrope.
Meanwhile you have market speculation that would use in part a forum like this as a sounding board for some kind of consumer sentiment index.
The complaints with GitHub seem fairly incidental, people airing their grievances on the incumbent because the cost of moving is considered either a hassle or a big-deal. But moving is an option, and the perceived stagnation is building a better competition (that they fulfill the promise without other expenses is always the gamble). GitHub isn't without problems, and it does seem like some obvious community complaints that have stagnated, but once released it'll probably just be a case of 'finally, thanks, no love lost'.
In the end, Git by nature is decentralized, easily self hosted, and both GitLab and Phabricator provide interesting open-source environments. It's not exactly a one way street.
But maybe it was always going to be a tough market to corner? My prediction.. more posts on HN describing migration to a different system and how it solved everything.. and then the followup 12-24 months later. Oh well.