I have no inside information, but from what I understand Sessions was a really good choice before he was appointed. Once he got the job, he did a complete turnaround. This is why Trump said, 'He is scared to death'.
Somebody or some group got to Sessions, maybe the Hillary mafia, and told him to stand down. There is no way to know what they threatened him with, but it must have been something very strong and very scary. Now, he is afraid of his own shadow. He does nothing, as he ordered to do by whomever, and just sits in his locked office all day playing Tetris until he decides to go home.
The Sessions we see today is not the same guy he was. Somebody scared the shit out him. What they said or did we will never know, but it worked. That is why he will never resign. He is not 'allowed' to resign. His controllers have ordered him not to, and so he obeys them.