US Constitution

Article I, Section 3, clause 6:

The Senate shall have the sole Power to try all Impeachments. When sitting for that Purpose, they shall be on Oath or Affirmation. When the President of the United States is tried, the Chief Justice shall preside: And no Person shall be convicted without the Concurrence of two thirds of the Members present.

Chief Justice Roberts isn't presiding though, so (other than the constitutionality of the proceedings in the first place) this raises some interesting questions

Roberts knows that he is not legally bound to preside over the impeachment trial since the wording of the law is very specific and Trump is no longer the President.

Even so…

What would make Roberts choose NOT to preside over this?

The biggest, most controversial case involving a President, and the Chief Justice says nah?

Even using the Democratic viewpoint that this is a completely legit proceeding?

Strange, no?

Maybe he knows the entire thing is unconstitutional and wants no part of it, which would require some level of integrity

For him to suddenly have integrity at this point in the game?

One might assume he's already flipped

I cannot think of any other scenario in which he wouldn't preside