#4 There's legitimate heat between Riddle and Goldberg
Matt Riddle has been clear of his distaste of Bill Golberg's in-ring ability. He made that abundantly clear after Goldberg's match with The Undertaker. He just believes that Goldberg has "terrible matches" and hurts his opponents. It was also reported that Goldberg blocked Riddle on Twitter. Matt Riddle was outright burying Goldberg, despite his legend status.
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The great thing about professional wrestling is that it blurs the line between real and scripted. That's what makes it enjoyable and draws in the viewers when they can't tell the difference. Based on this, a match between Riddle and Goldberg would only be best for business. Maybe, WWE is saving it down the line and probably Dolph Ziggler was chosen because he can work well with almost anyone.
But the story was already brewing for a while and it would have been the right way to launch the King of Bros into the main roster.