I suspect it largely depends on whether the PhD is expected to be printed as a book. In some countries (like the Netherlands) it is. Many printing services offer ISBN registration as a freebee. This is likely how most dissertations end up with an ISBN (I know mine has one because of this).
Other than this, I doubt many graduates go through the trouble of registering manually for a PhD. I don't know of any place that explicitly requires it. soSo in short, it's not expected, but it's not that unusual for a thesis to have one.