If R' Schneur Zalman of Liadi had long Peoth, as in this famous sketch, why is it not the custom in Lubavitch circles these days for adult men to maintain long, emphasized Peoth as it is in many other Hasidic groups?
The Lubavitcher Rebbe said that the custom among Lubavitch was to cut the Peos.
He said that there are a few reasons, one of which is to avoid mixing the two types of light from the 13 strands of the beard. He also mentioned that the Arizal used to cut his Peos (as is written in the Shaarei Hamitzvos and Taamei Hamitzvos parshas Kedoshim).
Years ago, I was told that the Chabad Chasdidim wear short peyos because there had been, at one time, a law in Russia forbidding peyos. They then started wearing short peyos, which they continued wearing since that time.