Vol'jin's proven to be a capable leader with a good head on his shoulders, and basically lead the charge. He's also been shown to have the Horde's best interests in mind. He didn't trust Garrosh from the start, and was right to. Honestly, he's objectively the best choice, both from a story perspective, given his buildup, and a character perspective.
And OP, if you really think Aggra would be a better choice, just stop playing the game. I'm really sure you're not serious though.
And to the people going "They promised us something unexpected" who the fuck did you expect? Rexxar? Gallywix? Nazgrel? Characters with no development throughout MoP when the big mystery the expansion built up to was who would lead the Horde? You've gotta be kidding me. It's basic storytelling, that you don't just drop something in last minute.
The literal only choices based on that, we're Vol'jin and Lor'Themar, and Thrall if they wanted to be terrible and backtrack. And Vol'jin was by far the more developed choice of the two. Some people, I swear.