Why does cultured marble yellow?

Typically the wrong cleaners are being used. Cleaners with ammonia, etc.  

Sometimes, the surface can be restored, but you will have to defer to a professional for evaluation, and in most cases if the yellowing is advanced, it will be hard to tell whether the time spent is worth it.

This means that more likely than not, a professional will suggest replacing the cultured marble top.

The reason for this is that the typical gel coat thickness is between 20 and 25 millimeters.  (roughly 0.787 inches) less than inch. 

Once someone sands through this, it renders the surface virtually useless.