That is actually a great question. I apologize for not answering it in my last response.
As bus speeds and processors become faster through Moores law the old hard drive wiring became the bottleneck. 160 MB Ultra SCSI-3 was not fast enough. Also, as you increased the number of disks being used to increase server capacity into multi-terabytes the distance between the hard drives and the bus became to large for SCSI technology. They therefore went to fibre channels to increase bandwidth and distance. (Notice I fixed my spelling. It is fibre according to SGI.)
As a side note, when you look at the speed of IDE drives today (ATA 100, 133, and 150) people often assume that these speeds make SCSI obsolete. This is actually not the case. The advantage to SCSI is that much of the machine's I/O load is offloaded to the SCSI processor. This frees the CPU to perform other activities. You will notice this when you transfer a large file (a few hundred MegaBytes) across a hard drive. The OS becomes very sluggish because the CPU is having to manage every byte being transfered. (To notice this on Unix machines you have to move the file across partitions, since Unix is smart enough to just move the inode assignment.)
Now for neural design. In very large servers you purchase processors in groups, usually four, and not in single units. This is because those processors are designed to work much as your brain does, calculating how to balance the load between both them and the other CPU clusters. (Sorry clusters is a term I made-up. SGI calls them bricks.)
If you look at http://www.sgi.com/origin/3000/datasheet.html
(click on the top link) and glance through the 3800 specs you will get a quick overview of large server class technology. I choose SGI because they own Cray.
calls their fibre channel a Fireplane, boasting a 172.8 GBps peak bandwidth, and also sell their processors in groups of 4.
Now I don't want you to think I have slighted the free OS's by what I have said. It is just a matter of economics. When a machine costs millions of dollars you just don't have time to port Linux to the machine. Capitalistic principles say it needs to be in production quickly. SGI, Sun, etc. have no reason to spend their time porting Linux. Since 8 and 16 processor machines have dropped in price Linux has been ported to them.