Passage: However, the two Panzer divisions of the XLVII Panzer Corps—after using their mobility to isolate Bastogne, continued their mission towards the Meuse on 22 December, rather than attacking Bastogne with a single large force. They left just one regiment behind to assist the 26th Volksgrenadier Division in capturing the crossroads. The XLVII Panzer Corps probed different points of the southern and western defensive perimeter in echelon, where Bastogne was defended by just a single airborne regiment and support units doubling as infantry. This played into the American advantage of interior lines of communication; the defenders were able to shift artillery fire and move their limited ad hoc armored forces to meet each successive assault.

Question: Whey did the leave one regiment behind?
to assist the 26th Volksgrenadier Division in capturing the crossroads.