> But why a 386? Why not a 486

This question can go on and on though. Why a 486 and not a P1? Why a P1 and not a P2?

If I had to guess I'd assume it was purely based on part availability, or cost.

The question seems to be why ship Windows95 preinstalled on a platform that can't run it, when highly integrated x86 clones such as the Vortex are available? https://www.vortex86.com/products/Vortex86DX

I assumed that was for the FAT32 and USB support. The stock image just boots up in MS-DOS 7.1 mode and you have to type “win” to go graphical.

Availability. Check out the datasheet on the 'CPU' in the article.

Also top 486s require a heatsink and a fan.

That was true in 1989 but modern x86 reimplementations like the Geode GX1 are much lower powered. The GX1 is discontinued but DigiKey still has > 100k of them in stock...