There are most likely a number of reasons but I think the main ones are:
1. Victims are ashamed. The whole nature of bullying creates low self esteem and damaged self confidence.
2. Many people find it easier to confide in someone who does not know them. Someone unlikely to judge or make the situation any worse than it is, which is a fear of many victims.
3. There is a lack of understanding within education establishments. Schools are afraid of admitting a bullying problem, therefore are likely to ignore the problem when confronted with it. If a victim has tried to tell a teacher but not got anywhere, a helpline is often the next port of call.
Female Respondent - North of England