There's a big difference between an attorney for the defense and an attorney in an impeachment. It's fundamental to criminal trials that a defendant, when put against the power of the government, and with life and liberty in the balance, to be able to present whatever defense is deemed necessary, even for defendants eventually convicted of the most heinous crimes (BTW, OJ did it).
In an impeachment trial, all the defendant is going to lose is their job. Life an liberty are not at risk. The power of the government does not overwhelm the defense, they are on a much more equal footing. Lastly, even a political process has to get down to, at some point, principle and respect for how a democracy works (unless all has already been lost).
The prosecution in a criminal trial has to be devoted to justice, not just convicting the defendant, because the prosecution works for the people. In an impeachment, everyone is working for the people, and so should be devoted to justice, as silly as that sounds in the current environment.