Some people might be better able to use their imagination when they read a book than when they watch a movie. Those people might prefer to creatively think about a story instead of being forced to see it in a certain way
Some people can get more emotionally affected by books as they describe things in detail. Those people might fail to get the same emotional arousal from the movie (See how to read a book fast).
As a person spends hours reading a book he becomes more connected to the characters and can easily identify with them. Because movies are short it might be harder for some people to relate with the characters.
Some people prefer to read a book than watch a movie because they are interested in the small details a book can offer (See how to read 52 books a year).
Some people find movies so stimulating and prefer a calmer way such as reading to know the same events.
When a movie about a book is released the plot can be changed and many scenes get removed. Some people believe this makes the movie less rich than the original book.
Because movies usually show things from a certain angle some people prefer books over them as they prefer to interpret the events in their own way.
Books usually describe the thoughts of the characters in details. Some people prefer books over movies because they allow them to understand the characters more deeply.
Because books rely on the writer's imagination while movies depend on a budget movies can sometimes fall short and fail to express what's in the book properly.
If a person likes a story so much he might want it to take a longer time. Those people might prefer books over movies.
Some people find reading to be an unpleasant activity. The people who prefer books over movies are usually among the ones who already like to read.