The issue is due to the ** pattern being used in MANIFEST.in. Although it seems to indicate "include any level of directories", it doesn't. It is just an equivalent for *, so ckanext/**/templates doesn't match ckanext/reclinepreview/theme/templates.
I myself remembered ** as "recursive match", but the documentation doesn't cite it either (http://docs.python.org/2/distutils/sourcedist.html#commands -- last paragraph) so I guess I remember incorrectly.
I'm opening a PR with a fix for this