Worth noting that "happy holidays" is not something used colloquially outside of America (to the best of my knowledge). Sure, we understand the context, but it's not something English speaking Europeans say to each other :) I'd be hesitant to suggest that alone is the cause, but might be an interesting additional data point.
It's not really used colloquially in the US either - if someone says "Happy Holidays" to you, it's because someone in charge of them told them they aren't allowed to say "Merry Christmas".
(Standard disclaimers about anecdotes and data apply.)