In Australia, it was because until the naughties, only the rich had central air.
In the southern states you might have forced air heat, but AC was rare. So you had split systems or window units to cool parts of the house. I grew up in a house that had a late 70s Carrier split system, we used fans to distribute the air around the house. That house was built in 1962.
Our current house had one split system when we bought it. We now have three compressors and 5 head units to heat and cool the whole house. Our place was built in 1992
Mums house had no AC or heating at all until about 5 years ago. We could’ve put ducted or splits in hers, but it was cheaper overall to use split systems and she just turns on the units that are needed. Her house was built in 1967
Michael’s mums had no AC until 2005 and then we installed a split system that heats/cools 2/3s of the main part of the house. As she’s getting older, we’re spending more time up there and working from there, so we’ve just gotten estimates to fully air condition the house. The first company can only offer split systems due to the Truss spacing in the roof being too tight, we’re still waiting on the second proposal. This house is built in 1976.
In the last 20 years, most new houses have central air now, however that has become a necessity because of small block sizes and houses lacking eaves or verandahs.
Split systems are a cheap way to retrofit to existing houses, or where you only need to control the environment in part of the house. Here they tend to last 10-20 years with limited repairs. Window or through the wall units here are seen as something used in commercial situations or when you can’t afford anything better.