Other people mentioned the real preemptive scheduling — and the general overall better worst-case latency — but another factor was the clean design. The other operating systems tended to be okay in the absence of I/O content but once you hit the capacity of your hard drive you would find out that e.g. clicking a menu in your X11 app did a bunch of small file I/O in the background which would normally be cached but had been pushed out, etc. A common mitigation factor in that era was having separate drives for the operating system, home directory, and data so you could at least avoid contention for the few hundred IOPs a drive could sustain.