Can't be sure of the reason but I believe it's heavier to pronounce. In the Quran at least, when بين is attached to a pronoun first it's repeated before the other word like in these verses:
My guess, but I can’t be sure as I did not look it up, it might have to do with the grammatical case. The pronoun is مجرور so it can neither be ضمير الرفع هو nor ضمير النصب إياه.
I understand that there is no consensus about العطف على الضمير المجرور بحرف الجر, but this is مجرور بالإضافة, and up to my knowledge there are no Classical examples that allow it - unless there is something I don’t know of course.