One theory—popular with the denizens of Fark—is that overprotective parents have ruined trick-or-treating. See, for instance, this article and the related Fark thread. However, this doesn't seem to be correct: no less authoritative a source than the National Confectioners Assoction reported in 2005 that 95% of children intended to go out. I would be interested in hearing reports from people in other parts of the country about their flow rates.
Another explanation is that it's a collective action problem: It's only worth trick-or-treating when enough houses dispense candy to make it worthwhile. [I've written about this before]. Similarly, it's only worth dispensing candy when enough kids come by: I was invited to several Halloween parties and stayed home to give out candy, but I'm not sure I would do that again. So, if you have a neighborhood which is in a no-trick-or-treating equilibrium, it's hard for it to take hold. I'm sure there's some effect here. For instance, Belvedere street in San Francisco has a huge party every year with the result that kids come by from across the city and residents put in huge amounts of effort decorating their houses. On the other hand, when I went out at 5:30 to pick up some more candy (ending up with way more than I needed), I had to stand in line at the register, so obviously people are giving out candy, and that means I don't have a good explanation.