The mind wanders in the wrong direction
We humans are masters of self deception. We like to keep ourselves busy not to remember our problems or face them. When some people read they lose focus and their minds starts to wander randomly. As a result some people get reminded of what they were already trying to escape from.
Some people are Afraid to be punished
This might sound weird but things will become clear when you know that reading reminds many people of the unpleasant time they spent studying for school exams. Our brains can associate a certain activity with a certain emotion. Because of their unhappy past many people have associated reading with unpleasant emotions.
The I want it now Attitude
Before the internet age people had to go to libraries to look for information but as information became readily available so many people developed the "I want it now" attitude. Because most books can't deliver information fast so many people started hating books.
Lack of patience
Most people are impatient, that's a fact. Modern technology has also helped greatly in reducing people's attention spans and in making them even more impatient. Long ago people weren't bombarded with content like they are today and because of that many grew impatient and lost their ability to stay focused.
Because cool people don't read
The human mind also makes associations between activities and personal identities. During high school so many people mistakenly develop the belief that the ones who read are geeks and that the cool ones don't. Even after people grow up those beliefs remain there and prevent them from ever reading a book.
Because they don't know what reading can do
Reading is the act of unlocking information. Whether you are looking for a way to get over someone or a method to start a successful money making business you will need to read. People who don't see reading as a method than can give them guidance to do whatever they want in life are very likely not to read.
They are afraid to find the answers
This might seem weird but in fact it's a very popular reason that prevents so many people from reading. So many people have developed a hesitant approach towards life. Because they are not brave enough they fear to take any forward step and as a result they avoid anything that can give them clear directions such as reading.