Because users and community contributors most likely already have an account, are familiar with the UI. There is also the "gamification" aspect that GitHub hav

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.