We haven't forgotten anything -- except you left out an important step.
After Rosenstein laid out his recommendation for the firing of Comey, and after Comey was fired, President Trump -- for reasons that baffle the hell out of me to this day -- did an interview with Lester Holt on NBC and made the astonishing statement that he fired Comey over the Russia matter. This put Rosenstein in a position where he looked like he conspired to have Comey fired under false pretenses. This is why he immediately announced the appointment of a special counsel to oversee the investigation.
Like I said ... I can't imagine why Trump would have been dumb enough to do what he did. Before that time, he had spent two years rightly ridiculing the crooked biased, fake news media in the U.S. as an arm of the Democratic Party ... and then in the days after firing Comey he finds several hours in his busy schedule to give a f#%&ing interview with NBC News? I know how those interviews work. For a 15-20 minute interview segment, NBC probably had two hours of interview footage. That crooked corporate media wing of the DNC was only too willing to edit the interview and present the most damaging parts of it to a national TV audience.
I love Trump, but in this case he was an absolute moron. This is the risk you run when you have a TV clown in the White House, I guess.
How do you know why Rosenstein did what he did? So the President gave an interview and made an ill-considered off-the-cuff statement. So what. If Rosenstein had an issue with the statement he could have addressed it privately with his boss (the President) and the communications office could have issued a follow-up clarification. This sort of clarification is routine and has been done countless times by all administrations.