They forget that they have died, or they forget that they can go home to God. Sometimes, they are encouraged to reincarnate by spirits that wish to take advantage of them. There's a strong tendency for falling into darker and darker realms, and to hurt others in the process. Once one begins to fall, it's easy to keep going. Think of the downward spiral of an alcoholic or a criminal.
Dark influence is a powerful, destructive, and yet subtle thing.
It is all too easy to become the victim of dark spirits. Such spirits darken the host. Worse, most people can't tell they are carrying such things and don't notice the effects. Even those with psychic abilities often can't find all the hitchhikers attached to themselves. Dark ones lie .