What happened to the attic?
Based on a purely random set of observations over my lifetime, I've noticed that houses more than about 100 years old (built before 1910 or so) usually have an attic which is fairly accessible for storage. Houses less than 60 years old (built after 1950 or so) usually have attics which are difficult to get into, or even completely sealed from the living spaces.
My own home, built in 1984, has at least three distinct attic spaces over different parts of the house, and only one of the three has any way to get in at all (without cutting through a wall or ceiling). Getting into the one accessible space requires carrying a large stepladder up to a closet on the top floor, lifting a drywall panel out of the way, and shimmying through a small hole--not at all practical for storage.
I find this a little mysterious. Attics are terribly useful things: they don't take up any living space but can provide an enormous amount of storage (think of all the billions spent on mini-storage); an accessible attic makes it much easier to inspect the condition of the insulation and look for roof leaks (and every roof, given enough time, will eventually leak); and attics are almost as handy as drop ceilings when trying to pull network cables.
So why doesn't the modern American house make it easy to get into the attic, the way our grandparents' houses did? I have some theories:
Beginning with the post-WWII housing boom, builders felt the need to put up lots of houses cheap, and eliminating the ladder to the attic was an easy way to save money. Eventually people stopped expecting this amenity.
Modern architectural styles, with their shallow roofs, don't give any usable attic space anyway, so builders stopped providing access. The trend carried over even in places (like Minnesota) where most houses are still built with a steep roof because of the snow and ice.
Building codes stopped allowing a steep ladder into the attic, and people didn't want to take the floor space needed for a proper staircase.
When people started heavily insulating their attics, it became more difficult to provide a hard floor suitable for walking and storage.
Attic storage is, and always has been, an expensive amenity reserved for the fanciest houses. Cheaply built houses from 100 years ago have mostly been torn down, so it just seems like builders used to provide more attic access.
Attic storage is just as common in new houses as in older ones, and my observation is just wrong.
My guess is that the answer is a combination of 1 and 2, with maybe a little of 3 and 4 thrown in. I really don't know, though, and my attempts to use Google-fu to find the reason came up blank.
So for now this is just a mystery. But if I ever build my own home, I will insist that it come with a proper staircase to an attic where I can keep all my stuff.