Because flavor/taste is subjective and easy to influence. While more people prefer Pepsi in a blind test, the results are not the same if people know which is Coke and which is Pepsi.
At some point, it makes more sense to invest in other products in the market than continue a never ending struggle to overtake one of the most recognized brands in the world. Tasting better just clearly won't do it. You would have to put an enormous amount of marketing dollars around it.