Some people don't check social networks like Facebook very often. It could be that they didn't login and that they will accept the request sooner or later.
When a person gets too many requests, they might overlook one of the requests. If a person doesn't check Facebook often and gets too many requests, by the time they check again they might only see the most recent requests. See also why people block others.
Many people don't allow someone to get close to them or see their private information if they don't know them well. If you sent the request too early, the person might not accept it. Some people don't trust others fast.
A large number of people, especially women, don't accept Facebook requests of people that they don't know in real life. If you send a friend request to someone without knowing them, they might ignore it.
If you make it obvious to someone that you like them and they didn't accept your friend request, it might mean that they are not interested in you. See why do people unfollow others on social networks.
Sometimes people don't accept friend requests of the people they are interested in order not to show them that they are interested. In such a case, not accepting the friend request can be a positive sign. See signs that show that someone likes you.
Some people, especially women, decide to accept friend requests later on in order not to seem easy to get. Those people will accept the friend request but after some time passes.
A person might not accept the friend request of the ones they don't like. If your friend request was not accepted, it might be possible that they don't like you. See why some people dislike you for no reason.
Some people get annoyed by Facebook accounts that don't have a real picture or a real name. In such a case, they might never accept the friend request of that person because they don't know who they are. See why some people don't put pictures on social networks.
Many people, especially women, don't accept friend requests of people they consider stalkers. Some people might not be comfortable allowing a stranger to see their pictures or know their news.
Many people only check the requests of the ones who have common friends with them while ignoring most of the other requests.
Famous people might have already exceeded the friend limit on Facebook, which is 5000. If a person has exceeded the limit or if they are close to doing so, they might be very selective when accepting requests.