I think a lot of valid reasons have been mentioned but I think the online aspect is a very big part. Cyberbullying can go on 24/7, where physical bullying can be avoided at times. Physical bullying is typically done by the bigger kids, cyberbullying can be done by anyone regardless of physical attributes. So the number of potential bullies is larger, and their impact is larger.
Don't know if any of you have seen where someone gets mad at their gf, posts nude photos on 4chan, then 4chan works to track down the girl, email the photos to the parents, the school, etc. They can rally an army (not all the time, but sometimes) of people who enjoy making someone's life a living hell.
If a kid finds great self-esteem in online networking, and finds that a sea of people are targeting him/her for abuse, I can see how that would be very difficult to deal with.