The wheels were designed with minimum weight to reduce stress on landing gear deployment motors and on total payload by 10-20 kilos, they are 20" in diameter. 

The wheel material was a bad choice: aluminium has a fast fatigue fracture potential compared to other metals. It doesn't elastically rebound into shape but instead it memorizes stress and microscopic fractures at stress points accumulate rapidly over time. The lifetime of a good steel frame bicycle is 10 times longer than that of an aluminium one. So aluminium is not a good choice for wheels. Spirit and Opportunity had 10" Alu wheels which were obviously more rigid and less burdened.

The aluminium between the ridges is .75mm, the contractors who made them must have known the wheels were not suitable for long distance rocky terrain. 

The small stones on mars are called "ventifacts" are sharp like walking on smashed roof tiles. The design team had not expected to have such sharp ground and the rover was designed entirely for sand and pavings.

Here on earth no one would ever use .75mm Alu for a volcano/desert rover to do 50km. It is s simple oversight/lack of testing. We use cable reinforced rubber, kevlar foam, Alu is just about **the least suitable** wheel material for a cross country rover because of it memorizes stress and it's lack of springiness. 

Tests have proven total faliure at 8km of all rock terrain, and limitless lifetime on sand.
Opportunity rover covered 42km in 11 years so they will still be able to run Curiosity for many years with limited terrain choice and it should still be a very successful mission with a very long lifespan.

The design criteria of the wheels at the time of launch were publicised to be it's good traction on sand and the distance measurements using the grooves, and the JPL written in morse code in the grooves. The design mindset seems to have misjudged the surface of mars to be strangely softer and less earth-like than it actually is, and to have not done distance testing on rocky surfaces. That's born out by the fact that the wheels fail after only 8km of rocky terrain. That's how long they take to be split in two all the way around.