Is there a known reason why it is not customarily for Chabad Chassidim to take Shabbos in early?
In Shulchan Oruch, bringing in Shabbos early is regarded as a noble practice. Nevertheless, in 770, Shabbos was never brought in early, and it is common practice in Chabad not to bring Shabbos in early. [It is told that Tzach once suggested that the Pegishos bring in Shabbos early to make it easier for the students who were tired and not accustomed to a late Friday night schedule. However, the Rebbe responded something to the effect that to bring in Shabbos early is not Minhag Chabad.]
Possible suggested reasons for this practice:
1. The Alter Rebbe writes (in his Teshuvos) that it has become customary to be stringent and not daven Maariv before Tzeis, and the Tzemach Tzedek writes that someone who is particular about this should do so even at the expense of davening with a Minyan before Tzeis. Even so, it is not completely clear whether their words apply to Friday night or just other nights of the week.
2. Bringing in Shabbos early requires one to adhere to the relevant associated Dinim, such as davening Mincha before Plag and Maariv after Plag, repeating Shema after Tzeis Hakochavim, reciting Kiddush and beginning the meal more than half hour before Tzeis Hakochavim. Included in this is ensuring that one has the right times for Plag and Tzeis, as there are different ways of calculating it. According to some Achronim, the husband’s Kabbolas Shabbos will influence when the wife has to Bentch Licht, but it does not clear that the Alter Rebbe would hold this way, and there are also a number of other reasons to be lenient.
3. When a Kehillah brings in Shabbos early, this can enforce Kabbolas Shabbos on individuals as well, subject to a number of conditions, which may inconvenience their Shabbos preparations or impede their chance to daven Mincha. This will also be detrimental for those people who are not yet Frum. [See שוע”ר סי’ רס”ג סעיף י”ח about having concern for others]
4. Not bringing in Shabbos early allows one time to learn Chassidus after Shabbos comes in, as a preparation for Kabbolas Shabbos and Maariv. [בדי השלחן סי’ ע”ז ס”ק ט”ו]
5. Some Achronim explain that the Maalah of bringing in Shabbos early is primarily when it is done for Hiddur Mitzvah and Kovod Shabbos, and not out of convenience. For example, if it is done year round instead of just in the summer.[ אגרות משה או”ח ח”ג סי’ ל”ח, באר משה ח”ב סי’ ט”ו]