Because they allowed the Clinton organization to buy Opposition Research, feed it to the FBI, and attempt to influence the election by sowing falsehoods.
Because hillary clinton is possibly one of the most corrupt and ideological individuals ever to hold political office, and the Justice Department will not investigate her.
So we have no russian collusion (and we never will have). And Clintons collude with everyone, in particular, George Soros against the united states on a regular basis.
https://www.quora.com/Why-are-the-Republicans-trying-to-destroy-our-Justice-Department-when-most-of-the-top-officials-are-Republicans-Seems-like-the-Republicans-are-eating-their-own-people