The writers of the NT were Jews of the Diaspora…Greek speaking Jews…the LXX was translated into Greek for the Diaspora who no longer used, spoke, or read Aramaic or Hebrew. Since the NT was written in Greek by Greek speaking Jews…what translation do you think they would have used?
There is nothing “magical” about the LXX…nothing “sacred” about the translation…it simply was the only translation of the Hebrew scriptures those writers used.
We now have the scriptures translated into various languages…with manuscripts from the Aramaic and Hebrew…why not use the Hebrew and Aramaic texts that the Hebrew scriptures were originally written in? Any translation looses “something” of the nuances of the original languages when translated into a different one.
Jesus did not speak Greek to those in Palestine…nor did his disciples…they were Hebrew/Aramaic speaking Jews of Palestine. When the Greek writers of the NT had Jesus quote the Hebrew scriptures…which translation do you think they would have used? A Greek translation of the Hebrew/Aramaic Jewish scriptures. Nothing of a mystery or conspiracy in that.
Most Bible translation use both and compare the translations…they even use the Latin Vulgate to determine what the various translatons are the closest to the original Hebrew and Aramaic…why would the OP think the LXX was any better a translation than the Hebrew and Aramaic? The originals were not Greek…so why not seek to convey the nuances of Hebrew and Aramaic into English by many translations.
Surely you don’t think Protestant translations ONLY use the Masoretic text?
Perhaps the OP should read the notes of the Revised Standard Version of the Oxford Study Bible…altenate readings from the LXX and Latin are in the study notes…as well as mentioned in the preface.