I think he was just next in line.
Exactly right.
Trump had to rely on recommendations just to get off the ground and turn on the lights at the White House. After all, there was an unwritten “stay” placed on anyone and everyone who had good experience in the Beltway, if they dared go to work for Trump.
There has always been a ladder usually of three at the Justice Department, who are eligible and meet the Department criteria’ to be moved into Deputy positions.
The tiny pool of choice at JD and FBI was a concern to Trump during the transition. It was discussed how small the pool was.
Now we see that Rosenstein was moved up to Acting Attorney General, the next one who moved up the ladder behind him retired, like last week, and the third I don’t know about.
This news of today simply says that Sessions knew Rosenstein from the old days and recommended him.
Trump deferred to Sessions, given that he sure as hell (himself) didn’t know Rosenstein.
Sessions, as a US Attorney was fired in the Clinton Purge, which was one of Bubba’s very first acts when he came in the door of the WH. Sessions was acquainted with Rosenstein from those years, presumbably.
Trump had minimal knowledge of who the black hats were in these stinking departments, or even of the white hats, during the time he was throwing together a staff and a Cabinet, early on.