Religion!!! Some people love talking religion, and others cringe at its mere mention - not that they're not religious, but no one wants to be Non-P.C. (Politically Correct). This has to do with the religious aspects of Halloween...
This article has various interviews about religious views on Halloween: http://www.religioustolerance.org/hallo_re.htm
So, we remember that in the past, the Celtics celebrated Samhain which was the last day of their calendar and one that they believed was the time when the veil between living and spirit world was thinnest. They celebrated the harvest and held various festivals. The Romans held two festivals around the same time, and when they took over the Celts these festivals seemed to have combined.
Now, it's reported (if my reading is correct) that the Catholic Church started All Saint's day on November 1st as a way to bring the Pagans (Celts, Romans, etc) more aligned with the Church.
As the holiday evolved it became less and less about religious celebrations, than a commercial holiday where children got candy from going house to house.
There are still groups today that celebrate it as a religious holiday, for example, the neo-pagans and/or Wiccans celebrate Samhain as a religious holiday.
Each religion seems to have a stand on the celebration of Halloween (If you followed the link, then you should have a better idea).
For more information about Samhain: