That's because courts are afraid of anything that threatens their authority. As a member of a jury they cannot make you vote in any particular way.
If you stand your ground they can call a mistrial, but they cannot force you to convict.
That’s true for members of the jury, not for attorneys in open court. Attorneys can’t say Jack crap about nullification during proceedings. Liable to be disbarred. Whether it’s all true or not his entirely irrelevant. Judges don’t want to hear anything about it.