Why are ad hominems not allowed in honourable debates?

Ad hominem fallacies are considered to be uncivil and do not help create a constructive atmosphere for dialogue to flourish.[35] An ad hominem attack is an attack on the character of the target, who tends to feel the necessity to defend himself or herself from the accusation of being hypocritical. Walton has noted that it is so powerful an argument that it is employed in many political debates. Since it is associated with negativity and dirty tricks, ad hominem attacks have erroneously been assumed to always be fallacious.[36]

Eithan Orkibi describes two forms of ad hominem attacks that are common during election periods. The first is the precedent ad hominem, according to which the previous history of someone means that they are not fit for the office. For example: "My opponent was (allegedly) wrong in the past, therefore he is wrong now". The second one is a behavioral ad hominem: "My opponent was not decent in his arguments in the past, so now he is not either". These kinds of attacks are based on the inability of the audience to have a clear view of the amount of false statements by both parties of the debate.[37]

Criticism as a fallacy

Walton has argued that ad hominem reasoning is not always fallacious, and that in some instances, questions of personal conduct, character, motives, etc., are legitimate and relevant to the issue,[34] as when it directly involves hypocrisy, or actions contradicting the subject's words.

The philosopher Charles Taylor has argued that ad hominem reasoning (discussing facts about the speaker or author relative to the value of his statements) is essential to understanding certain moral issues due to the connection between individual persons and morality (or moral claims), and contrasts this sort of reasoning with the apodictic reasoning (involving facts beyond dispute or clearly established) of philosophical naturalism.[38] "

https://en.wikipedia.org/wiki/Ad_hominem#Improper_usage