From what i have read it was a joint project between West Germany and USA to make some some seemingly over-engineered MBT. But why was it cancelled? It seemed fairly interesting and promising project, albeit with a sizeable amount of problems, that surely could have been worked out during the years?
Whipping out my Hunnicutt....
The MBT-70 program was supposed to be a tank which shared logistical requirements, making life much simpler between the US and Germany. Unfortunately, it became something of a design-by-committee problem. Even the question of "Should the thing be built in metric or imperial measurements" had to be resolved at the SecDef level. There is a long-standing history for that, take the M1917 tank of WW1, which was a variant of the French FT. Parts not compatible, and the tank is slightly differently sized. Anyway, all this management by committee and compromise led to expense and consumption of time. Worse, the Germans wanted a tank for Europe with a high velocity gun, the US wanted a tank more suited for general purposes, and armed with a missile. The Germans wanted the crew in the hull, the Americans tried for everyone in the turret. The Americans wanted a lightweight Continental air-cooled diesel, the Germans preferred a water-cooled Daimler-Benz. Eventually, the tank ended up being about four tons overweight. The Americans didn't mind, they preferred to have protection. The Germans minded, they wanted their tank to be within the weight limits for their roads, bridges, flatcars and low-loaders.
In the end, you just had so many competing requirements for differing national priorities, they may as well have had separate tanks to begin with. Plus the costs were going through the roof. The project was nixed by the Americans in January 1970. The Germans went on to build Leopard 2, and the Americans went on to the XM-803 (AKA the MBT-70 Austere). That only lasted two years before the Americans decided it was still a little too expensive, that the missile launcher may have been a bad idea to begin with, so they nixed that and started the XM-1 program.