It might not be around as long as FITS, but isn't DjVu more suited for the digitization of manuscripts?
The Vatican isn't choosing FITS because it's more suited towards digitization of manuscripts. The church intends to be around literally forever and they're choosing FITS because it too, should be around as long as there is SCIENCE! From the FITS wikipedia article: 'FITS was designed with an eye towards long-term archival storage, and the maxim once FITS, always FITS represents the requirement that developments to the format must not render invalid existing files using older versions.'
Imitation is the sincerest form of plagarism.