unlink(2) can't unlink directories except as root, plus you have to follow certain specific rules. rmdir(2) can. This is a historical thing.

(You must manually unlink(2) the '.' and '..' entries along with everything else in the directory before you can unlink(2) the directory itself. except on systems where this functionality was completely blocked in order to make you use rmdir(2). This behavior is how directories worked in 7th research edition and earlier, and AT&T System III; the syscalls were added primarily for the benefit of NFS so that directory creation/removal would be atomic and not involve root permissions vs. root squashing.)