Primarily religion, at least in Europe.
My home country ~1000 years ago used a runic script but when we adopted Christianity our scribes started using Latin, of course written with Latin letters.
Many countries in Europe used Latin as an “official” language, some even up until the 1800s. This meant that official documents related to matters of state and church were written in Latin.
Literacy wasn’t very wide spread in the middle ages, but of course some people who knew how the write wrote in their own language but they did so by adapting the Latin alphabet to their unique pronunciations. This was most likely because they learned to read and write in Latin in the first place. Since they all used Latin as a base that’s why most letters are the same across European languages.
You can see this same process in the East where the Byzantine Orthodox church was in power, leading to many Orthodox countries like Russia, Bulgaria and Serbia using the Cyrillic alphabet today, an adaptation of the Greek alphabet to better suite Slavic languages. Later this was spread by the Russian Empire to countries like Kazakhstan and Mongolia.
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