Shouldn't it be "horror filled halloween" or "spooky halloween"? It fits the purpose of the day. Why "happy"? By the way "Happy Halloween everybody!"
The wish for a "Happy" Halloween is a wish for the person to enjoy the day, regardless of how little or how much spookiness they wish for on that day.
Likewise, "Happy Christmas" is a common expression in the UK, wishing for an enjoyable Christmas.
Though it is true that people attempt to wish one another a "Spooky Halloween", this simply hasn't caught on in popularity, and "Horrible Halloween" would be similiar to telling someone to "Have a Rotten Day!", the connotation of you wishing them a bad day is just too overpowering to make it work.
The alliteration of "Happy Halloween" also likely helps keep it in its place as a popular greeting and well-wishing for the day.
Because people say happy "everything" now. It is like a drone sentence that gets repeated by the non-thinking masses. Happy Monday. Happy Thursday. Happy Columbus Day (I heard this a couple weeks ago). Happy roll back the clock weekend!
This might be the masses or it could be Hallmark. If you put the word "Happy" in front of any phrase then they can sell millions of cards for each "Happy" phrase. Happy answer.