Why can’t he argue voter fraud?
He was charged in the impeachment of "Incitement of Insurrection". The case at hand is whether or not he did that. Arguing election fraud is:
A. Admitting he incited insurrection as a result of his being cheated.
-or-
B. Not pertinent to the fact that he didn't incite insurrection.
Any good lawyer on the other side would argue that it has no relevance to the case, and the biased judge would throw it out as inadmissible.
John Roberts himself recused from this, knowing that it was unconstitutional and a circus. Trump and his team should do the same. As I mentioned, his presence only gives the charade credence and will not change a single vote for or against.