That's because arena 3s is not new, nor are BGs. Both have been tested in the past and (get ready for this) the cold hard truth is that arena is viewed only by a rather small group of players and that "true skill" is tied to arena performance, yet the large majority obviously does not share that opinion. If people loved the arena concept and mechanics then they would play it.
I can not say why they dislike arena and favor BGs etc, but my guesses would be the following:
1. Arena is repetitive and dull, especially at higer levels. Same patterns over and over - in BGs it is the wild west.
2. Difficult to reach high in rankings because only a few certain classes/specs shines. In BGs any spec can shine and turn the tables depending on the enemy. And since the selection is so large and random it can allow less powerful specs/classes to play and still win.
3. Finding a good arena partner is problematic. Not only do you need to find someone to sync with but they must also play the right class, spec, have the skill, have a headset etc (if u shall go high). Some players just prefer to rely on themselves and not someone else.