Because they can’t show the judge has been biased in his rulings in this case. And organizational or political affiliation is not grounds enough legally unless there is a specific conflict of interest or prejudice such as parties to the case are the judge’s relatives or when the judge owns stock in one of the parties etc. Honest lawyers that file a motion they know is BS are in danger of losing their law licenses.
An interesting case study:
In 1998, some attorneys (one of whom was Larry Klayman, the founder of the conservative advocacy group Judicial Watch) were dissatisfied with a ruling made by a Second Circuit judge, Denny Chin, and thought it wise to send a note asking about the judge’s Asian American background and then filed a brief questioning the judge’s impartiality. The judge was not amused. Here is how he responded:
(The attorneys) were required to withdraw as counsel from the case and would not be permitted to appear in Chin’s court on any matter ever again. They would be required to show his opinion to any other judge in the district in any future case. The court clerk would also report the sanctions to every court where they held bar membership.
They should have never questioned Judge Chin’s impartiality.
They should have questioned his sanity.
Maybe that's the answer. If Trump were to become his own lawyer in the case, I don't think that the court could short-circuit his claims by punishing his lawyer.
Perhaps Trump would be pushing this too far by representing himself in the case. But, it might be interesting and it might bring things to a head.