No one CHOOSES to be deeply affected by bullying. No one *wants* to be bullied so viciously that it becomes part of their depression. No one *lets* it affect them like this. This is akin to saying "I don't know why she let her skin bruise like that after they punched her". It's involuntary. Words can hurt us just as much as physical acts. They can cut so deeply. It's well and good to wonder why she didn't stop using twitter but why the hell should she? For every troll that harangued her there were messages of support and love too. Should she, or anyone, forgo this support and let the trolls win?
She had so much trauma in her life and struggled so hard- these Internet bullies were just one factor in a series of things that must have pushed her over the edge.