Sure we got a spirinling ball.. lets add some WINDI'm thinking it was wind. Kakashi did say he wanted to put some chakra into it, and the only thing that would make sense is to put wind in a swirling ball
Fire? who the heck would use fire..
Fireball sure is something so rare that only most genius would think of that..
I never heard of FireBall, Lightning Ball... wind ball? most common thing ever.
Sorry but WIND is not the most obivious, Fire and Lightning are the most common anywhere.
Ah! But this is naruto! well then this strengten even more my point, Wind is a rare element, as stated by Kakashi or whoever. (Only asuma has afinity with it... know anyone else? Maybe Gaara's sister)
Also as Kakashi explained, Yondaime created Rasengan because that is the apice of Chackra manipulation, not because (Lets create a spining ball to add wind on it)
He first created the ball, then he tried to add whatever element to it, he did not create the ball thinking on any particular element.