The Gadianton judges have managed to make Nephi a source of controversy rather than curiosity or even sympathy. The audience is no longer listening to Nephi but arguing among themselves. Nephi has lost his forum.
Mormon, unsurprisingly, concentrates on the positive arguments for Nephi, even though there must have been just as many counter arguments from those who sided with the judges. However, Mormon is not summarizing pros and cons in an objective way. Rather he presents Nephi in the best light.
When Nephi sees sufficient support in the crowd to retake control of the situation, he begins to speak.