The reason why the Event Pokémon codes are long is because it needs to write all the data when you receive the Event Pokémon. If the Event Pokémon codes were to be short, you would receive a Bad Egg.
The reason why most people are receiving Bad Eggs is because the Event Pokémon codes has incorrect data or the data is incomplete, but a few Event Pokémon codes out there will have typo errors in them or will not be converted completely and thats why a lot of people are receiving Bad Eggs.
For example, the Movie Arceus posted by @ PokePalace is not converted completely. The line [E0009F50] should be [E0009E4C]. The Movie Arceus with the line [E0009F50] posted by @ PokePalace would be for the Japanese Version and by switching the line [E0009F50] with [E0009E4C], it will now be converted completely and thus be for the US Version.