Why is suicide increased in professional althetes who have had repeated head injuries?

Probably depression. Brain injuries are often associated with depression which in turn is associated with suicides.
Suicide. Certain head injuries can result in fluctuations of mood, where depressive phases are so severe that the person is at higher risk of suicidal behaviors. They simply cannot endure the pain anymore and feel there are no alternatives.
Bitter fruit. A professional athlete devotes effort into excelling, far beyond what most of us put into anything. The reward may or may not be a pile of money, but a person who senses the brain is damaged cannot enjoy either success, fame or wealth. The damaged mind may be unable to sustain loving relationships, satisfying work, or enjoyable activities. Try to understand ; support efforts to prevent this.