It makes sense, if one considers them the evolution of the Apple II and the IBM PC.
The IBM had the motherboard on the left, along with the expansion cards, and the drives and power supply on the right. The AT continued this.
Clones wanted mostly compatible cases, and motherboards wanted to be mostly compatible with IBM cases and clone cases.
Then we had Amigas like the Amiga 3000, which had a similar layout but a riser to take horizontal expansion cards.
While some more bespoke PCs had vertical risers, most PC cases in the early to mid '90s were large. It was the machines we paid a bit more for that made being smaller in to something a bit premium.
While taking apart my Amiga 3000 is a bit of work, the design is absolutely wonderful, and more than once I thought about the design of it compared with later machines like the Sun Ultra 5, the Motorola StarMax (PowerPC Mac clone) and others.
The Ultra 5 (desktop) and Ultra 10 (tower) were a cost cutting exercise that put an UltraSPARC IIi (2i) onto what I think was an ATX form factor motherboard. It used ATA drives, USB keyboard and mouse, a VGA port, etc. This was an act of desperation from Sun, not an example of their best engineering.
That said, compared the performance of a $3500 Ultra 10 with 512 MiB of RAM to $10k+ Sun Ultra 30’s and HP C180’s, each with 128 MiB of RAM. These prices were after applying significant edu discounts. The heftier sheet metal, SCSI drives, and nostalgia did not allow these traditional UNIX workstations to touch the performance of the much cheaper Ultra 10 with 4x the RAM.
The IBM had the motherboard on the left, along with the expansion cards, and the drives and power supply on the right. The AT continued this.
Clones wanted mostly compatible cases, and motherboards wanted to be mostly compatible with IBM cases and clone cases.
Then we had Amigas like the Amiga 3000, which had a similar layout but a riser to take horizontal expansion cards.
While some more bespoke PCs had vertical risers, most PC cases in the early to mid '90s were large. It was the machines we paid a bit more for that made being smaller in to something a bit premium.
While taking apart my Amiga 3000 is a bit of work, the design is absolutely wonderful, and more than once I thought about the design of it compared with later machines like the Sun Ultra 5, the Motorola StarMax (PowerPC Mac clone) and others.