Because my guru says so! 😘
Ok, feeling generous today, so here´s the long form:
The middle finger represents Tamas Guna(pls google if needed),
so in my tradition you do not touch the third eye with it,
instead, we bend the middle and index finger till they touch the palm,
so they are out of the way, for Nadi Shodana.
But, beyond all doctrines, it is always best to experiment with different approaches
and then stick to the ones that work for you.