That's a good one and the answer should proove that most cat food company out there are not to be trusted.
The reason is simple : Cats are not the one buying the food. It is proven that people buy cat food according to their own image of what their cat should like. So mouse-flavored food is not as appetizing as chicken or beef.
Another reason is that cat don't taste the same way as we do. Their tongue don't have the same area of taste as we do. Bitter taste for exemple is not located at the same place and it is much more pronounce for cat. That's why they really don't like the smell or taste of lemon. Their brain is also not trained to recognize mice meat as coming from a mouse. In the wild, cat don't hunt according to their taste but rather to their needs and what they can find and catch.
If presented with two options, your cat will go for the one which taste better (more flavoured, more fresh and most of all, the one that smells the most). So your cat will not prefer mouse flavored food over chicken because for him, it's just another taste. Cat don't like mouse because they taste good, they love them because they can hunt them and catch them.
By the way, I will not stop repeating it but ALL food sold in supermarket are the equivalent of Fast food for your cat. Most food sold in petshop, although better, are not so good either. Read the ingredient and avoid anything that has "meat by product or Flour of meat" or any derivative. Also check on the internet for lawsuit action against a company before buying their product. You will be surprise of the results. Keep in mind that the bigger the company, the more money they have to make you believe they are better....(I.E. Royal Canin)
Cat behavior consultant and working in as art director/marketing in a design and printing company....