Now one thing that Github has is integration. You get lots of stuff integrated for free where Atlassian offerings are separate products. And you don't have to configure much.
You can't compare Github to just bitbucket. You have to compare it to a combination of bitbucket, bamboo with Elastic agents and Jira. With a competent admin this is all easily set up and administered. And especially if you are a largish company Jira is the killer vs. Github issues. Especially if said company is somewhat process heavy which Jira is just the best at. So configurable and extensible. Of course a small startup or open source project will prefer the zero fuss Github.
And of course as devs we don't care much about those parts. But you did ask why they are still alive.