The main reason SCSI isn't used much any more is cost.
For the usual purposes (hard drives), SATA works nearly as well and is much cheaper.
The other use of SCSI is to tie multiple (commonly, up to 7), high performance, intelligent devices together on a common bus. And that is also like SATA. You can usually a less expensive way of doing that.http://en.wikipedia.org/wiki/SCSI
The big advantage is that SCSI is a well defined standard.