Linguist here.
I could write something up, but something tells me that it would offend 90% of the posters here and probably not worth the flack I would receive. BTW, there’s a whole branch of linguistics devoted to this.
That being said, in short – without offending anyone’s beliefs: For any reading of Hebrew, look at the surrounding languages and their evolution. The surrounding languages caused the emergence of the Jewish vernacular in said area. Any evolution in those languages – including that of the Jewish vernacular – changed the phonology of Hebrew in that area.
With the exception of the rednecks from Eastern Europe, it’s basically agreed that the Teimanim are considered to have the most “authentic” reading of Hebrew.
Also, I contest the statement that the Litvish reading is original American – the Yekkes and Dutch Portuguese were in the US before the Litvaks and Chassidim.