Why is it easier to submit a bug report if my bug reporting system is run by the same company as your code repository? Why are those things even slightly related?
Because users and community contributors most likely already have an account, are familiar with the UI.
There is also the "gamification" aspect that GitHub have. Doesn't motivate me personally, but could have effect on some others.
Projects on GitHub gets a lot more visibility.
To the point that many projects that do not use GitHub as their main forge are still often mirroring their repository there, and have to deal with double source of bug reports or pr.