Isn't that the problem, though? GitHub has one Issue tracker that is "good enough" for most projects I've worked on, and gets better over time because GitHub dog foods it (even if they don't always see some of the large open projects issues with it). Bitbucket has two Issue trackers, a "sort of useful" minimalist one that is worse than GitHub's and an upsell to Jira which is maximalist overkill for any project I've worked on. Atlassian doesn't have much incentive to make Bitbucket's issue tracker any better than it's current "almost decent" because it wants to upsell Jira. You can pretty much assume that Atlassian only works in Jira themselves.

(Similarly, VS Team Services has only one issue tracker and I'd rather use that than both of Atlassian's offerings, even though it's almost equally maximalist with Jira, mostly because they clearly dog food it and don't try to upsell into it from a worse system that nobody wants to use.)