Why do people say "Happy" Halloween?

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.

"Happy" is just the generic modifier for holidays. I can think only of Christmas as an exception to the rule ("Merry Christmas").