as chip was saying, theres less sideloading on straight cut gears, normal gears are helical which is when the teeth are cut at an angle, like -> /////

because the teeth are at an angle, when you start chucking massive ammounts of torque through them, the two teeth in contact with each other try and slide away from each other, puts strains on bearings and shafts and if they do start to move appart, the area that theyre incontact with each other gets smaller and they can strip.