Because piling on all the related bad stuff in a single case is more effective than piecemealing the related crimes. If you can convince a bunch of jurors that TMFWSNBN did criminal fraud with the fake electors conspiracy, it’s a lot easier for them to conclude that he agreed to have the Proud Boys and the MAGA mob fuck up Congress’s shit in the conspiracy to obstruct. Vice versa too. Basically, the more related bad conduct, the better for the prosecution.
Wire fraud, however, would be a distraction. Fleecing his online marks doesn’t have much of anything to do with J6.