Because colors with components that result in the same intensity will be identical. Think a shade of red and a shade of blue that wined up the same shad of gray. The fact that both were distinct colors is lost.

By limiting the transformed colors to those a person is blind to, you limit the amount of information loss.

I think the ideal place to tackle this issue is as near to the display as possible. A screen purpose-built for the color-blind would be best, but redshift-esque software would do the job just as well.