Answer: Because private companies will have difficulty getting anybody to pay for them.
Step-by-step explanation:
Private markets fail to provide the optimal amount of some good such as public firework displays because private companies will have difficulty getting anybody to pay for them.
The main motive behind private markets are simply for making of profit but for public, the main motive is for the government to satisfy the needs to the people. Hence, with regards to the question, the answer will be that there'll be difficulties encountered in getting people to pay for the goods.