ABS sensors can be a real bear to remove due to the corrosion that can happen in the sensor bore. I physically destroyed the last one that I removed (the coil was open-circuit so it didn't matter) due to corrosion holding it in place. YMMV
Apply a rust penetrant, wait, and then try to get the sensor to twist or move even the slightest bit. Repeat this action as more penetrant is added, and eventually it should come out.