• Ownership of IP (Intellectual Property) may restrict the number of vendors. Cost of licensing IP may make alternative vendors uncompetitive, so they don't enter the market. Single source is not wise in most (but not all) cases.
  • Cost may be beyond the point of diminishing returns for the majority of applications, just as the cost of SCSI is beyond the point of diminishing returns for most desktop PCs, but priced just right for midrange servers.
  • Lateness to market may make marketing cost to achieve economical volume too high, limiting the product to only the highest end market.
Products are designed for particular markets. IDE was designed very deliberately to reduce cost in volume production. Put the controller on the device, and make it a simple controller. SCSI was designed for greater flexibility and better performance. SSA was designed for very high end systems.

You can put 2 IDE devices on a cable, 16 SCSI devices. Even with just two, IDE devices don't share the cable very well. Hell, when I put a fast CD writer and a CD-ROM in the same machine I have to put them on separate IDE cables or I get underruns making copies, and CDs aren't really all that fast. I don't have that problem with SCSI.

True, newer IDE drives outperform older SCSI drives (at least if there's only one IDE drive on the cable), but SCSI is a moving target. Soon we will have Serial IDE which will solve the "2 per cable" limit, allow longer cables and greatly improve data transfer speed, but Serial SCSI has already been in development for quite some time and will be there for the more demanding markets.

Right now, high performance IDE is operating system dependent because of the special drivers needed. SCSI isn't. You can get high performance out of SCSI even on OS/2. I seriously doubt I could get SSA to work at all with the small business servers I build.

True, many SCSI drives are mechanically the same as equivalent IDE drives except for the control circuitry, but many aren't. Different SCSI drives are targeted to different markets. Right now IDE doesn't have that differentiation - it's all aimed at the lowest price market.

Trying to force any of these devices into the others' markets is not going to work well.