Self-storage centers were certainly not unknown in the early/mid 90s (around here in NY, they were mostly converted factories), but in the last 5 years it seems that the number of storage centers has exploded (both single and multi-storey - the single story ones tend to have garage-type door entrances) - it looks like at least one is build every month (and these are not converted factories - they are custom build). Names such as Shur-guard (with the cool lighthouse), Westy, and USA storage are now seemingly everywhere in my area (in at least one instance, two large storage centers face each other across the road).
Why the suddenly increase in available storage space - have life-styles changed so much in just 5 years that we have all this extra stuff that needs to be stored (Mine certainly hasn’t)? Is this a bubble that will soon burst (leaving us with dozens of empty big-box storage space and acres of empty individual garages)?
I think there are lots of reasons,such as:
Cost of living causing some people who previously lived in houses to move to apartments. Not all their stuff would fit in an apartment, and rather than get rid of it they store it.
‘Sandwich generation’ - Age group of people 25-45 ( i think) who are both raising children and caring for elderly parents. Storing their parents stuff when they move to nursing homes, or storing it after inheriting it after the death of a parent.
Also, maybe newer homes in your area are being built with less storage space, attics basements, etc?
I know that except for cost, I’d use one. I just had to move from a 2-story townhouse to a 2-bedroom apartment. After a week and a half, we still haven’t made a dent on the roomfull of boxes in what should be my daughter’s room.
Stuff accumulates, and absolute stuff accumulates absolutely. 
It’s a new mentality: before, you’d buy an appliance, you’d get it fixed when it broke, but always just have the one. Now, it’s just as cheap to buy a new one sometimes, yet something in us compells us to keep the old one “just in case” you can get it fixed cheap. I’ve got 4 TV’s, one is a new 19", one is an old 14", another 19" with the blue color gone, and one last 21" that has a bad power switch.
Also, sentimentality. We save our childrens’ first booties, their first birthday cake decorations, their deflated mylar baloons from when they came home from the hospital.
Finally, the “things will change” attitude". My wife has a lot of her “skinny” clothes that she’ll someday fit in again. We have a corner computer desk that won’t fit in our new living room, but we’ll keep it for when we have a LR that it will fit in. We also have a full-size freezer that’s in our storage unit until the day we move to a bigger place.
We’ll probably concede to sell most of our stuff, and start over when we do get a big place again. I take it as a sign from God that we have too much stuff.
The apartment I moved from had a walk-in closet in the single bedroom, a large pantry, a hall closet, a large laundry room, a linen closet and a storage closet on the porch.
My condo has an outdoor storage closet (at the end of the building), a very shallow hall closet (shared with the hot water heater and the air conditioning unit), and a shallow closet (about two feet deep).
Most of the houses in this part of Florida do not have basements, and of the new houses and plans I’ve looked at, most sacrifice storage space for extras like a large family/gathering room, a den/children’s playroom, or a pool. The garage is large enough to open the car door without dinging it (maybe). Most of what is laughably termed ‘attic’ in the real estate ads is more of a crawlspace that would give any claustrophobe the willies.
Several of the self-storage places now have units large enough to store automobiles and boats. (Great for when the homeowners association gets on someone’s case about too many vehicles in the parking area.)
Suffice to say, we have tend to have too much stuff, and no where to stuff it for free. Now we stash it for a price.