If you compare the ASTM specification sheet (the properties list) for gasoline and diesel, diesel seems to have more properties that need to be protected by additives. There certainly are more diesel fuel additives in the marketplace, both multi-functional additives and the single function treatments that fuel suppliers are required to use to ensure  the diesel they're supplying performs the way it legally should. This may be due to the fact that gasoline is more highly refined, where diesel is a heavier fuel and so is more prone to going "off-spec". Lubricity, cetane rating, cold weather performance, fuel stability - All of these are essential properties that have to be protected by additives.