Because the ray-traced renderer (the only thing in AE that really uses CUDA much) isn't seeing any further development. You can tick the box to never show that message again. If (for some strange reason) you do want to use the obsolete ray-traced renderer, you can go into your settings and choose to enable "unsupported" cards and AE will use your GPU to accelerate the ray-traced renderer.
But, seriously, there's not much reason to use the ray-traced renderer anymore. There are more powerful options in AE now (and some great training on how to use it).