Rudy was a part of the Trump campaign. As such, he is possibly someone whom Mueller would be questioning. Trump’s firing of Comey put Trump in a position where he cannot plug a guy like Giuliani in to AG. And, despite the Trump fans who’d support him, as he condescendingly said, even if he shot someone on 5th Avenue in NYC, an Attorney General must be approved by the US Senate. The US Senate is not going to approve Giuliani and risk the wrath of their electorate. Despite the hubris that the Trump victory has caused in the minds of many pro-Trump supporters, most Americans still believe that the rule of law matters more than the passing fad of Trumpism.
Almost as many Americans still hope that the Hard Left tyranny of Mueller, Rosenstein, Comey, Hillary Clinton, and Barack Obama is also a passing fad.