I disagree in part with the snitch theory. Bullied students are afraid. It's not that they would rather be bullied than "snitch". They are afraid that if they do tell: 1)teachers won't do anything; 2)if teachers do anything, it won't be harsh enough and then the bullying will get worse; or 3)that teachers will make them sit down face to face and discuss the situation with the bully. These are three real reasons, but not the only reasons.
No one likes a snitch. And whether that is a silly statement to make in the face of real and sometimes harmful or dangerous bullying, is irrelevant in the face of the reality kids face, which is: no one like snitch.
Kids want to succeed in their peer groups, they want to be respected and liked, so much so that it sometimes overrides all personal considerations, including personal safety. They would rather be seen as sticking up for themselves than needing the help of an adult. Or in some cases, they may simply be too ashamed (rightly or wrongly) to ask for such help.
In my opinion, students don't report bullying to teachers because they do not want to be seen as snitches who would go against their fellow students. This has been part of the unwritten rules of being a teen at least since I was a teen and I think that it continues today.
I also think that students are afraid. They think that they will get bullied next (or, if they're already being bullied, that the bullying will get worse) if they tell. They think that teachers might somehow reveal who has turned in a bully and then the bullies will take revenge on them. Teachers can't be everywhere and so this is a real fear.
I too would say that there is a fear factor. These students might fear that the bully will retaliate. However, there can be other explanations. Some students might feel it is pointless to report bullying to their teachers because attempts to do so in the past did not solve the problem. There are many teachers who turn a blind eye to the bullying that goes on in schools. I think students quickly learn that "tattling" is not something you should do because the teachers say so themselves. I think this conditioning happens from an early age. Young kids learn to find ways to handle their problems and they internalize many of their feelings.