That's because Branden comes from a variety of places most of which take their name from the English elements of brom 'broom' and dun 'hill'. Branden and Bradon thus occur but only rarely does the sound of -dun inspire the use of an a for the u. Many people would also find the double A's a bit 'much', preferring the balance of Brandon or Branden just as they prefer the visuals of Brennan to Brennen.
You can write to Mike and ask him to add your spelling as a variant however and then maybe you would get to see your exact name here.
Devon