I'm surprised that nobody has mentioned that the vast majority of Halloween costumes and paraphernalia is manufactured in China. That gives them a pretty wide exposure to it.
It's a pretty small manufacturing sector. I doubt many Chinese are exposed to Halloween just because the costumes and masks are made there. If that was a good reason, then the Chinese would be into every non-food item you see in your retail stores, 'cuz just about everything is made in China, now.
And why are they manufactured there?
Because America outsourced them there.
Well..... long before everyone began sourcing everything in China the folks in the Halloween "business" sourced their goods in China. The trick to Halloween products is that like someone said above about sewing costumes, they don't have to last but a day or night. Cheaper than cheap is the route to go. If you want a high-end costume of Louis XIV, for Madonna's next "Bal Masque", go to a costumer. There are still some. But the stuff they sell in America through the month of October is pretty much disposable crap.
I know all this because I'm buddies with the King of Halloween from the logistics perspective. Americans will be able to guess who it is, but previous NDAs force me to be coy. I handled their Halloween supply chain. They have fifteen hundred own stores and another 1500 franchises. That's 3000 stores; which are door-to-door Cheap Halloween Crap, as he called it, from Oct 1 to 31. The day after? It all gets thrown away/recycled/donated. As the holiday has gotten more public, there's more and more stuff sold every year.
And, surprisingly, it's not just for the cheap. It's for the inexpensive but efficient factories. There is a tight window of the entire array of ordering and quality testing, manufacturing, and shipping that the factories have to perform in about six weeks. They've had offers of cheaper product from Thailand, Bangladesh, the Philippines, Indonesia, etc... but the factories can't produce the high volume with any consistency in production and quality. They've moved all their sourcing back to China.
Sorry for the logistics derail.... it's in my blood.
As to the popularity of the holiday.... the most that kids do over here in Asia is have a coloring page to do in school or maybe a party with cupcakes. Trick or Treating is not a thing. It's an adult holiday, not dissimilar to St. Patty's Day or New Year's Eve or for those who are in those countries, the Mardi Gras/Carnival.