He is on video encouraging the entry into the Capitol.
Relevancy, and burden on the witness. Each individual defendant is accused of having entered the Capitol illegally, etc.. Even if he says "He, there was this other guy who encouraged me to go in, and I want him to testify", it doesn't make that defendant any less guilty, which is all that would matter in that criminal trial. Also, Epps lives out of state (apparently), so a judge in D.C. isn't going to be inclined to try to force him to testify in D.C..
And hey, I'd love to see the guy have to testify. I just don't see the pro-leftist judge handling these cases go along with it. You could argue entraptment, but that's hard to prove and again, this crappy judge would likely say there is no evidence that he was FBI anyway.
Just being realistic as to why we haven't already seen him called by one of the defendants.