There are several reasons why they were placed on tracked systems:
1. Mobility. AA gun systems were placed on tracked chassis to allow them to stay with the Tanks and Infantry Carriers they were designed to protect. Cross country mobility and floatation on soft/wet ground is much better with a tracked system versus a wheeled or towed system, and quicker to deploy while on the march. By removing a heavily armored Tank turret and installing a more lightly armored AA turret the system's weight usually goes down to the point that an older chassis design could keep up with the more mobile newer Tank and Infantry Carrier designs.
2. Survivability. The armor envelope (as thin as it might be) provides greater protection to the crew and the mechanical components of the AA gun system. Typically the greatest threat to these systems came from small Infantry and/or Anti-Tank ambushes or Mines...not aircraft. These AA systems usually spent most of their time traveling to/from combat rather than being engaged in active fighting against aircraft.
3. Availability. The armored and tracked chassis chosen were already in series production, but were starting to become obsolete in their originally intended roles as Tanks and Infantry Carriers. It was often much more efficient and cost effective to continue them in production rather than retool a factory to produce a new chassis design. Simply design and install a new AA turret and drop it into the existing turret ring or basket. Also, spare parts, POL (Petroleum/Oil/Lubricants), and trained maintenance personnel and procedures were already in the system and widely available and understood by the time these systems were chosen to be coverted to AA use.
CptA
