The most likely reason as to why Game Freak didn't make Ampharos an Electric / Dragon type in the first place is that the Mareep line was found rather early in-game, and Dragon was always considered a late-game type by Game Freak as a balancing factor. However, this is apparently not the case in XY, as the introduction of the Fairy-type means that Dragon-types can come along a bit earlier now that they are more balanced in comparison to the past generations, as seen in a screenshot depicting a horde battle against wild Axews that are around level five, and the fact that the newest Fossil Pokemon is a Dragon-type, meaning that Game Freak can now use their new method of Mega Evolution to finally retype Ampharos to fit its concept/Japanese name, and because the Mega Stones or whatever are hidden, they don't have to worry about someone obtaining such a powerful type early enough to trivialize parts of the game or whatever their concern was.