Its easier to make a game for casuals because it encompasses what everyone wants. Tailoring to only people who play all the time actually creates huge problems with the game for daily living.
Casual as stated far above does not mean easy as well. It just means giving people who don't play at least 20 hours a week, a chance to get good items and catch up with other people who beat normal difficulty. Often people who play casually still complete all the content of the game, just at a much slower pace.
Honestly its not even a game style, its just common sense on how to create a game. You don't want a game like WoW where once you commit to playing, you lose 8 hours of your day automatically after all.
In the end, a game made for casual people can be just as enjoyable for hardcore players as well. Giving hardcore players better loot and things like that will eventually lose the skill aspect of the game as well. Completely overpowering other players isn't really fun for both sides after all. The idea I believe blizzard is shooting for, is two people who beat the game and are around the same level. Should stand roughly an even chance in PvP. But perhaps someone who completes Act 1 Nightmare. Would just blow away people who only completed normal difficulty, but probably not so extreme as that lol.