Today is Yud-Tes Kislev, the anniversary of the day the Alter Rebbe of Lubavitch1 was released from prison. I once heard that his chassidim sold their silver ataros on their tallisos to pay for his bail, and this is the reason the Chabad custom is to not have an atarah on the tallis.2 What is the source for this rumor?
1. Rabbi Schneur Zalman of Liadi, also known as the "Ba'al HaTanya".
2. This is doubtful the reason why this is the minhag, but it's an interesting story nonetheless. I believe the real reason is because we don't want to distract ourselves from the real beauty of the tallis, the tzitzis, by which the entire tallis is necessary, with a flashy atarah. Or, as brought down in the Aruch HaShulchan (thanks @user sam) to not mistakenly think that the ikkar of the mitzvah is atifas rosh.