Mostly because of Nintendo. They priced the cartridges in a pre-downloadable gaming scenario, and haven't adjusted to the new reality...
Also, Nintendo believes that lowering prices diminishes the perceived quality of the product, so they keep the prices up to get the impression their games are on an entirely different level than anything found in the apple/android/XBLA/PSN store.
Also, Nintendo believes that lowering prices diminishes the perceived quality of the product, so they keep the prices up to get the impression their games are on an entirely different level than anything found in the apple/android/XBLA/PSN store.