[FreeReign]

... he refused to accept Rosenstein's resignation as U.S. Attorney for the Maryland District while all this was going on.

Rosenstein was the only "Republican" appointed holdover going back to the Bush administration, so he kept him.

Pretty straight forward.

[Alberta's Child]

Trump kept him because he wanted him to be Deputy AG. If Rosenstein resigned as directed (by Sessions!) and he wasn’t confirmed for another two months, then he’d be getting job offers from big D.C. law firms and would likely lose interest in the Deputy AG post.