There are a number of suggestions explaining this difference. One is the reading is the result of an effort to remove the concept of the Two Powers in Heaven that was seen as a threat by some. IOW instead of two powers, now there is only one. A second proposal suggests the opposite, that the MT (Masoretic) reading had been altered.
Johan Lust proposed the thesis that the current MT is, in fact, an “early Targum” of the original Hebrew text of Daniel, and that the OG translates, accurately, that original Hebrew text, now lost, in which the Ancient of Days and the Son of Man were, indeed, “one and the same symbol.”
In this case, the distinction between the Ancient of Days and the Son of Man would have been
introduced by the current MT – the Aramaic that supplanted the original Hebrew – in order to give voice to the apocalyptic-messianist agenda of that “early Targum.”