Because of how intensely popular the movie was when it came out. After a little while, it stopped being cool to like Titanic and became cool to hate it. A lot. Like, a lot a lot. Like, so much that it became part of popular culture and has reverberated so strongly throughout space and time that even you (despite your general non-giving-a-fuck about pop culture) have assimilated it and responded accordingly. That's why this entry is so defensive. Liking Titanic is STILL a good way to get yourself mocked out of existence even by people who have never seen it.