[wolf550e]

Hebrew had over a million speakers even when it wasn't anyone's native language. It was known to Jewish men who studied in a cheder as boys (those who did not continue to higher studies/seminary in a yeshiva), the way I think Arabic is known to Muslims who are not native speakers. Hebrew was used for literature and poetry and for communication between Jewish communities that didn't share a native language. It wasn't reconstructed from literary sources, it was only modernized for everyday use.

[nine_k]

The problem that there were several dialects, each having a significant number of speakers, both in medieval Hebrew and medieval Italian languages, to say nothing of Germanic languages.

It took certain development of compromise standards and getting used to them, when a single state appeared which needed a unifying language: Israel, Italy, Germany.

All these countries still have a number of dialects spoken casually, but at least there is a common standard to use when in doubt.

[azernik]

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.