1 The believed it's a scientific hoax
Some people didn't like Interstellar because they believed the scientific theories displayed in the movie have very connection with reality yet they were intended to make people believe in them.
2 They didn't like the people who claimed the theories are real
Some people disliked the movie because of the attitude of some others who claimed they know a lot about the 5th dimension , which is something theoretical, and that the movie is all about scientific facts.
3 They found it very boring
Thrill seekers and people who were expecting a lot of action found a very slow paced long movie. This led some of them to find the movie boring.
4 The Movie was over hyped
The movie was over hyped by the media and people. This created some king of high expectations that turned into disappointment for some people.(See Interstellar (2014) Movie Explained)
5 The ending seemed rediclous to them
Many people didn't like the ending of Interstellar as they believed it's unrealistic and far from being true.
6 Long movie few events
Many people found the movie to be very long. The fact that the events were slow to happen made the movie seem longer. (See Why some people don’t like movies?)
7 They didn't like the dialogue
Some people, and some critics, believed the dialogue was lousy and a bit lame.
8 They didn't like the ones who kept praising it
Many people hated the movie as a result of the people who kept saying that it was the best movie ever created. Those people found those reactions very exaggerated.
9 The sound mixing issues
Many people , especially critics, pointed out that the movie had problems mixing the sounds that in some parts they couldn't hear the people talking because of the music.