Eeeeh -- "invented" is a strong word. Alexander imposed Greek culture on his empire, and his language came along with it. Koine evolved from Attic during the epoch of Alexandrian Greek influence. It changed, as all languages do, with continued contact with regional languages. It was simplified and incorporated lots of loan words and neologisms. For a long time scholars called Koine the "Holy Ghost language" because the New Testament seemed to be the only documents from antiquity that used this dialect, but archaeological discoveries over the past century (e.g., in Oxyrhynchus in Egypt) show that Koine was just the common, everyday dialect of Greek as spoken in the classical world at that time. Some authors would deliberately revert to Attic for a more highfalutin tone (much as modern writers might mimic Elizabethan or Victorian English), but Koine was the language of commerce, of scholarship, of finance, and even everyday speech in many places. And it was very long-lived as dialects go - from its development in the 300's BC all the way to 400AD or so when the language changed to medieval (Byzantine) Greek.