I think the Commanche was cancelled because it was too expensive for a piece of equipment that was designed to fight in a Cold War scenario. The DoD (thankfully) realized that they didn't need a brand new, horrendously expensive piece of equipment when we already have weapons systems in place that can already effectively complete the missions the Commanche was designed for.
Basically, they wanted to put less money in brand new weapns systems, and more money into personal gear for soldiers.
(see "CRUSADER SP HOWITZER")