It is very simple. The tzioini havara is a man-made street language and adopting it for tefila means that you put the values of modern Zionism above proper tefila. Listen to Shwekey sing Al Homotayich if you want an idea of how (some) Americanized Sefardim really speak – it does not sound like modern Hebrew at all.
Do you really want to speak to Hashem in the same language that you use to order a shawarma at the tahana hamerkazit?
In fact I use the old Russian-Litvish pronunciation at the amud davka because that is not my daily dialect even when I speak Yiddish (which was a street language as well but now is spoken mostly in the frume velt). I call my friend Moishe, but at the amud it’s Mayshe uvnei yisroel lecho onu beshira.