The systems are not being replaced. The SADARM round (used in the 155mm guns with two warheads per shell), and the sensor-fused weapons on the CBU-97 cluster bomb and the AGM-154B Joint Stand-Off Weapon (JSOW) are almost as good as a nuclear weapon, and much less of a political mess. Since they are conventional weapons, they can also be employed to assist troops faster than the nuclear shells.
To put it simply, the nuclear artillery shells are no longer needed to wipe out a battalion or two of tanks, and so they are being retired. Instead, the infantry can call on supporting artillery and the Air Force to deliver a blow that is just as hard, and which wont have as much fallout in either the nuclear sense or the political sense.
The dismantling of the nuclear artillery shells also will free up a lot of resources for other projects due to the security measures required for the nuclear shells. The troops used to guard them can now be shifted to other missions.
The U.S. Department of Energys National Nuclear Security Administration dismantled the last of the artillery-fired tactical nukes. The dubious honor went to a W79 shell, normally fired from the 203mm M110 self-propelled howitzer. The W79 had a one-kiloton yield, and could be fired to a range of 30,000 meters with a rocket-assisted projectile.