Right now we can only comment on the code that changed in the pull request. However, there may be cases where

1) someone notes that a behavior in code elsewhere in the file (that wasn’t modified) may now be different with these changes, or
2) Changes elsewhere in the file (e.g. missing imports or incorrect constants) are required, or
3) A bug is discovered and someone wishes to make a note

I believe any of these, but especially 1 and 2, are sufficient reason to mandate this change.

Furthermore, this is a regression (for my team anyway) from Reviewboard as this behavior was supported on there.