This was done to improve consistency and avoid ambiguity. As explained in the commit that changed this behavior:
Working directory can be easily confused with the current directory.
So this change was made to better disambiguate between the working tree, meaning the location where your repository has been checked out, and the working directory where you are running the git status command, which may be somewhere beneath your working tree (or perhaps not, if you set GIT_WORK_TREE environment variable).