Because it was used for international communication (e.g. for da Volterra's travel memoir and rabbinical responsa), and so was standardized across a wide geographic area. For example, before the expulsion Ashkenazis would send legal questions to Spain for answers from prestigious rabbis, and the resulting legal rulings would be distributed across the Mediterranean and as far as Iran.
WRT the Italian and German cases, these are not really exceptional; standard French wasn't a common native language in France until a post-revolutionary homogenization campaign, Castilian Spanish still isn't universally a native language in Spain, etc.
Making an existing lingua franca into a more common native tongue is a standard and early step in the formation of a nation-state (in the old-world sense) from Norway to, less successfully, India. None of these phenomena set any useful precedent for the establishment of a conlang as an international language.
