Because of the TJ's coil spring suspension. Disco'ing the rear antiswaybar helps and is normal for leaf spring Jeeps that are laterally very stable but things go downhill from there with the unstable coil springs. The rear antiswaybar helps reduce the tendency to flop (tip) or roll on extremely steep or offcamber terrain.
Not to mention that disco'ing the TJ's rear antiswaybar does not help increase usable flex/articulation in the least. Disco'ing it gives no benefits like it does on a leaf spring Jeep, it only has handling and offroading-related drawbacks.