Sora got the keyblade because Riku don' fucked up, since he was supposed to be the original (which comes around full circle I guess.) Afaik, Sora called Ventus' heart from the darkness (or Ventus found Sora's heart) and they both accepted each other? I think that was before KH1 even. From what I can tell, Ven seems to be the reason Sora can do that crazy shit with his Nobody and dual wield. But as for being a Keyblade wielder, I'm pretty sure he was just the next best choice since Kairi was all doped up as well. Ven's influence helped since the trinity moved to Sora's group, but I suspect that's a coincidence and Sora always had what it took to be a master.