It's New York State Law that requires Trump's presence in the courtroom, not partisan political reasons. That requirement could have been waived, maybe slandering the judge and his daughter wasn't such a good move?

https://casetext.com/statute/consolidated-laws-of-new-york/chapter-criminal-procedure/part-2-the-principal-proceedings/title-k-prosecution-of-informations-in-local-criminal-courts-plea-to-sentence/article-340-pre-trial-proceedings/section-34050-defendants-presence-at-trial#:~:text=Except%20as%20provided%

He may yet be removed.