Short answer – coal and wood stoves.
Longer answer, is because we have reversed when we have houses open and closed, so to speak, at least in much of North America. Back in the 1800s and early 1900s, because it was so hard to heat houses, and summers tended to be cooler compared to today, when fall arrived, people shut the house for winter. Heat came from wood and coal, open or contained fires that produced ashes and soot. Plus people tracked wet dirt, mud, and ordure into the house. People didn’t do laundry as often in winter because of the difficulty of heating the water, making space for the wash process, and then where to dry things? Soot, dust, dirt, and other stuff built up over the months. Women cleaned as best they could, but come spring, it was time to open the doors and windows and clean.
The longer days, warmth, and brighter light also played a role. You could see dirt better.* Weather improved to allow work outdoors, and things dried faster. The time had come. Guys moved heavy things, locked up papers and anything that didn’t benefit from being tidied or scrubbed, and got out of the way. Everyone started with moving movables out in order to lift rugs and carpets and take them out to get beaten, brushed, and cleaned as best one could. Curtains often got similar treatment, and might even be put away for the season, replaced with lighter weight draperies. Then one scrubbed the floors and perhaps the walls, removing soot and ash. Stoves were empties and cleaned if possible, pictures cleaned and dusted, light fixtures lowered and cleaned, or at least the glass globes were removed and washed, the brass polished.
Laundry followed, whites, heavier things, whatever could be cleaned. The goal was to get all of winter’s grunge out of the house, polish, dust, and otherwise refresh things. Easter being a time of new life also played a role, but the need to de-soot the place was more important in many places.
Not every community did this. German and Dutch areas stayed as clean as possible at all times, because otherwise the neighbors would talk. Those of you who have been in the Netherlands and Germany, or lived in Dutch and German parts of the US, know exactly what that means. In the Deep South, cleaning was whenever needed, since often the weather stayed warmer and better for more of the year.
Today, it might be better to reverse things, and do the deep cleaning in fall, before we shut everything up for the season. Most houses are not beset with soot and ash the way they used to be, but dust, pollen, and other things come in during summer. Once the doors and windows are closed and latched to keep out the cold, allergens really become a problem for some of us. A good top to bottom scrub might be better than spring cleaning. Or do both, depending on where you are and what your weather’s like.
https://www.realsimple.com/history-of-spring-cleaning-8601487
https://www.countryliving.com/home-maintenance/a38381/how-spring-cleaning-became-a-tradition
*There’s an entire subfield of history about the down-side of better interior lighting, including the greater stress on women to keep houses cleaner, since dirt was literally more visible.
Cat Rotator's Quarterly 
Not just the houses. German mother or grandmother out scrubbing the stop and sidewalk are still seen infrequently. Woe betide the youngster who make a mess of it. This was common in days of coal or oil fired factories, with soot from them and vehicles – daily sweep and scrub.
When my folks were in the Netherlands, everyone was out on Saturday afternoon scrubbing the front stoop, tidying flower boxes, and making certain that things were just so for Sunday. Where I lived in [redacted] state was very, very similar. It felt a bit like being back in Germany, with a different accent.
My English Granny Dodie recollected to me how she and her sisters scrubbed the front stoop of their house, in the working-class (later slum area) of Liverpool late in the 19th century/early 20th. It was a matter of pride to her family. (Also – that she grew up a street or two over from where several of the Beatles were born, half-a-century later.)
Ha! Now expecting a vignette where something abyssal drips ichor where Lucy sees it on a clean walkway or a fence. Out comes the broom and maledictions in Clan and Lutheran. Betting on 300 feet and four planes. That’s if it cleans up its mess first.
The story of Silver shooing a beast off plane was hysterical. I can see other quietly fierce women doing similar.
With Jude standing there nervously.
At least Shadow knew how tough Silver is, but we haven’t seen Lucy in a magic fight.
And the problem of opening up the house right now is… Pollen and dust (NM your dirt is in my patio, come get it)…
We’re supposed to get a nice dose of NM and maybe even AZ on Tuesday. Come on rain!
Yep, we’re supposed to get hit Monday night…
Interestingly in Japan the big clean happens before the new year i.e. during the cold of December these days. In the past I believe Japan used the same lunar calendar as China and so the clean up happened in the cold of January (not much different)
>. Back in the 1800s and early 1900s, because it was so hard to heat houses, and summers tended to be cooler compared to today
Thinking is hard, isn’t it?
If you mean taking the end of the Little Ice Age into consideration as it relates to fuel use and weather, it can be if you are not aware of centuries-long patterns. If you mean in general, for some people it might be. If you intend the comment as a drive-by insult, I’ve heard worse.