With the Steelers missing their QB, Miami wasn't going to accept anyone else's first.
They may have if it had been offered when the negotiations started rather than after Ben was injured. I don't know if the Dolphins planned on waiting until after week 2 to see which season destroying injury drove up the value or if it just worked out that way.
I'm also not so sure this was a smart move by the Steelers. They may need that #1 to find Ben's replacement.