Because pull requests can be ignored as well, although it is true that they're more visible than a mail in a mailing list archive.
GitHub lets you update patches easily (and see the updated patch), drop comments inline in a patch (very useful for code review), and tag patches (extremely useful for large projects).
Having to go through the extra hassle of a mailing list just to have the maintainer ignore them sucks.
Another benefit of GitHub is that users affected by an issue that has been patched but not merged can just fork the repo and merge the PR themselves and use that (I've done that in multiple cases). Mailing lists aren't as easy for this.
This is true, which I discovered while attempting to contribute to git itself. git's development uses this model, which probably means something, even if I can't figure it out.
The vim development model (patches + mailing list) has been pretty frustrating for some people. That's one of the reasons why NeoVim was forked.