I guess it's because back in those days, there weren't as many threats or concerns about stuff like... y'know... pedophiles and child predators and stuff? I mean, when you think about it, for grown people to go around dressed in costumes and asking for candy does seem a little... "odd"... but they still do it, they just don't go to houses and ask for candy, they just go to conventions and pretend they're the characters they're dressed as, they call it "Cosplay".
And touching on Drtooth's point, yes, ever since 9-11, trick or treating has seen a huge downturn... apparently, more and more schools and churches hold parties on Halloween night to let the kids have their usual fun in a safer and more secure environment; as I said earlier in response to Sarge, there's also a really big fear about predators in this day and age... seriously, every year on the local news, a day or two before Halloween, there's ALWAYS a news story about, "Do YOU know if sex offenders are living in YOUR neighborhood? Let's take a look... John Doe lives a 1613 Road Street, two doors down from where you live, he's a registered sex offender, and he's not supposed to live here because this neighborhood is full of children, and he looks like he's ready to invite children over to his house for trick or treating. There he is, let's ambush him with obvious questions. Mr. Doe, you realize you're a registered sex offender right? Do you know you're not supposed to live in a neighborhood full of children? Why are you living here and why are you going to be letting children come to your house for Halloween? Mr. Doe, are you ignoring us? Why are you running away from us and locking yourself in your house? There's more predators like Mr. Doe living in your neighborhoods, and that's why Halloween is no longer fun for children, back to you in the studio."