Because GitHub only allows you to assign to one person per issue.
Wait really? I could have sworn this wasn't true... Must have gotten confused somewhere
1) jbrooksuk's answer, only one assignee.
2) Assignees are the person owning the changes, not the person reviewing.
3) There is no status associated with it, you can't say "checked" vs "unchecked", and certainly not unreviewed/accepted/rejected.