Bro is exclusionary to women because it is a word for a man or boy. So associating guys (and exclusively guys) with the name bro in this way is a small sign to women that they don't belong. I am sure this seems inconsequential to you, and taken by itself, it is. But as a woman, seeing literally (first definition) dozens of these signs every single day on everything related to technology, it adds up.

You, personally, only notice when something might be exclusionary when someone else brings it up on a site like this. But if you are sensitive to the signs (ie are either a woman or looking really closely) they can be overwhelmingly negative.