This.
DC still supplies some comic books for newsstand distribution (but it looks like they have a $3.99 cover price on the New 52 $2.99 books since last year), and Marvel doesn't even bother with that any more. But it's also a case where many retail stores don't always want to be bothered with comic books and have shrunk their magazine rack space in general. (Plus, if the comics they did get weren't selling, there gets to be a point where there's no use carrying them any longer.)
One other difference between then and now. When I was a kid, a few 20 or 25 cent comic books were doable with my weekly allowance . . . with comic books about $3.00 or $4.00 each these days, how many can a kid buy with a typical weekly allowance?