First, it is primarily an Anglo-Saxon festivity, based on Celtic pagan culture, thus many non anglo-saxon countries do not celebrate it.
In Latin origin countries, especially Catholic ones, there is another party, the All Saints’ Eve. In Catholic culture, witches are not celebrated… but it is instead a time to remember the loved ones who are dead.
However, with globalization, the Halloween celebration seems to be spreading. I can tell you about Brazil. Until the ‘80s, there were almost no local references to Halloween, except for Hollywood movies.
It seems the parties started at English schools, to bring students closer to Anglo-Saxon culture. Later, it spread out to higher class schools and today it can be found at upper classes and some thematic places, but is far from being spread out all over the country, it is still restricted to niches.
Halloween is seen by some as “imported culture”, not very welcome in a country rich in its own culture. Thus, some schools have been trying to replace the imported celebration by bringing back the local rich folklore, as Brazil has plenty of its own mistic dark creatures, a mixture between European, African and Indigenous culture.
I will disagree and say that a lot of western countries do celebrate Halloween at some degree. If you look for a US style celebration with trick or treat and costumes you will have a hard time to find them, but my point is that some traits of Halloween have mixed with the local culture or have so many points in common with it that they become intertwined.
Why do people say then that they don’t celebrate Halloween? Because of cultural pride and resistance to US cultural and economic imperialism. Its presence is undeniably anywhere, tho.
Let’s talk about Mexico. This October I went to a Halloween party as a lot of young people does now (costumes are not as important). The one I went to was decorated with papel picado (DdM) and blood on the walls (H). Día de muertos merchandise has become spookier by the year (scares are not part of día de muertos) and you can buy calaveritas next to plasric Jack o lanterns andlatex monster masks.
Kids also go trick or treat (no tricks tho) on the streets on November the 2nd (this is a mixture of old Hispanic traditions and I’ve read of something similar with the Aztecs, but now kids go dressed up as Disney characters and such due to US influence) and they go chanting “me da mi calaverita? (Would you give me my [sugar] skull?) with direct reference to día de muertos traditional candy.