Because fortune is an extremely powerful psychic power for an army(Eldar) with 3+/4++ saves. Giving it to an army(marines) that has 2+/3++ deathstars that already win major tournaments(adepticon) is retarded.
a 2+/3++ rerollable save is basically unkillable outside "remove from play" abilities. It doesn't help that Marine Deathstars absolutely crush any models with "remove from play" abilities either.
It also doesn't just affect deathstars, its just the main offender.