Because IDE sucks, and why bother giving it the good stuff? Pinto for you, poor boy.
And, I am being serious. IDE is not as good as SCSI. It can't take as many drives, it can't go as fast, and it cant access multiple devices at once. Not to mention that it isn't a bus like SCSI, all (both..) devices share a controller. SCSI can handle 15K drives because it can take enough of them to saturate a bus, a couple 7200's on IDE will get near maxing it out. Unless you really believe that you'll get 100MB/s off of ATA 100...