Why the Quran Was Really Revealed in Arabic, According to the Quran Itself
According to the Quran itself, the Quran wasn't revealed because all other revelations had been corrupted (to the contrary, the Quran confirms the preservation, inspiration, and reliability of the Scriptures of the Jews and Christians in Surah 3:3-4, 5:47, 5:66, 5:68, 7:157, 10:94), but it was revealed so that Arabs wouldn't be able to complain that they didn't have an inspired book in their own language when they are judged by God (Surah 6:155-157). The Quran says that other nations had been warned before (Surah 16:36), but that no one had yet been sent to the Arabs (Surah 32:3, 34:43-44, 36:2-6), so God sent Muhammad to the Arabs as the last prophet. So according to the Quran itself, Muhammad did not reveal the Quran because the other scriptures had been corrupted, but so that Arabs wouldn't be able to complain on judgement day that they didn't have an inspired book in their own language.
(side note: the Quran says no one had warned the Arabs, but this doesn't make sense, because the Quran also says in Surah 2:125 that Abraham and Ishmael were in Mecca and built the Kaaba in. So did Abraham and Ishmael not warn the Arabs?)
Another interesting point to be made is Muslims often say that to be able to see how well written and eloquent the Quran is, you must read it in Arabic, its original language. However, this is a big problem, because the reason the Quran was revealed was so the Arabs wouldn't have to learn a new language to read God's word, since the Torah and the Gospels were written in Hebrew and Greek, respectively, and not in Arabic. So the Quran was written so the Arabs wouldn't have to learn a new language and wouldn't have to trust what Jews and Christians said about the Torah and Gospels, yet that's exactly what Muslims say the world must do in order to see how beautiful and eloquent the Quran is, to learn a new language, Arabic. So the very things they say the world must do is the exact opposite of why the Quran was said to be revealed in the first place.