Because AG Sessions could not have investigated 'Russian Collusion' and appeared impartial. Collusion was supposed to implicate POTUS according to #FakeNews. So Sessions recused himself and let someone else take over. There was no Trump Russian Collusion. The entire investigation would fall into 3 possible outcomes.
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They would run into dead ends with no evidence against Trump.
2 They would fabricate evidence against him. This would eventually be discovered and would lead to a grand jury indictment.
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They would uncover the collusion with the DNC.
Any of those options was a win. Sessions recused himself, took the heat and has been running other investigations such as U1 behind the scenes.