I agree with Turtle, but it may also be the board where the thread is located. All the boards are different in popularity as well, along with the threads they hold inside. But just because a thread has no replies doesn't mean it hasn't been read alot c: I see some threads with 0 replies that have alot of views.
~ Whisper
I think the reason some threads are a tad more popular is because of ho posted the thread, and who are all the ones that write in the threads. If you think about it a lot of popular floofs tend to all post in the same area as the others.