Why do languages that adopted the Latin alphabet have very different uses for some letters/letter combinations?

I can understand entirely why different languages that developed using the Latin alphabet would have evolved different pronunciations. But when it was adapted as the written form of an existent language, why would the pronunciations change so dramatically? I'm thinking most specifically of Irish Gaelic and Welsh which seem to have very different orthographies than most other languages that use the Latin alphabet, such as "w" for the long "oo" sound in English in Welsh or "mh" in Irish Gaelic for what would be the "v" sound in English. Why wouldn't there be a more 1:1 association when the written language is adopted later?