They are busy, so busy
A person can reply with a short answer if they don’t have enough time to type long phrases or words.
The Person isn’t interested
If a person isn’t interested in someone, then they might reply with short answers.
To push someone away
A person can reply with a short answer to push someone away. This usually happens when someone nags or sends too many messages.
To end the conversation
Sometimes a person might use a short answer because they want to end a conversation with someone.
To show lack of interest
A person interested in someone might reply with short answers to show that they are not interested. This happens if the person has a certain strategy in mind. (See Why some people are not interested in others?)
They Can’t find the right words
A person can reply with short answers if they don’t know what to say. People who run out of words might reply with a short answer.
They are feeling bad
When a person is in a bad mood, he might not find enough energy to type or chat. In such a case, that person might reply with short answers. (See Why Am I in Such a Bad Mood?)
The topic is not interesting
A person could reply with a short answer if the topic being discussed didn’t seem that interesting.
The person is angry
A person can reply with short answers to show that he is angry. If someone is angry at another, then the earlier might send short answers to the latter to express his discontent.