If a person is not motivated enough to reply to a certain message he might ignore it on purpose.
If a person is not motivated to reply on an unconscious level then he might forget to reply to the message just because a part of his mind isn't really motivated to reply. See also why some people reply with short answers.
Some people read messages then decide to reply later. Those people might forget to reply to the messages they read earlier.
Some people read messages then don't reply to them because they can't find the right words to reply with. In such a case, being unable to handle the situation is the main reason that prevents those people from replying back. See why some people prefer texting over phone calls.
In some cases, some people don't reply to intentionally hurt others, make them feel bad or to get revenge. This can either happen consciously or unconsciously.
Some people might not reply to messages after seeing them in order to confuse the ones who sent them. Players and people who are interested in you might do that.
Sometimes a person might read a message by mistake without even realizing that he did. In such a case, the person can miss the message and so never reply.
When a person is busy he might not reply back to a message. Later on that person can forget about that message. Some psychologists believe that forgetting the message only happens when the person believes it's not that important.
Some people prefer not to text while driving. In such a case, those people can read messages and don't reply.
Some people read messages then try to think about the possible reply. In such a case, those people usually reply back after some time.
When a person is in a very bad mood he might read messages but never reply to them.
Some people don't like to get into conflicts and they might stop replying when they feel that a conflict is about to take place.
If a person is mad at you then he might not reply to your messages on purpose.
This is common among friends. A friend can be too lazy to reply to a message or type when he believes that he will see his friends later and discuss the message.