The early 1990's when Linus created Linux was the perfect time. And at that time all of the Unixes were walled off proprietary prison camps and ran on workstations that at that time cost a couple tens of thousands of dollars. Linux ran on a common PC.
I was running 386BSD on x86 PCs in 1992-93. While all UNIX wasn't proprietary at that point, this was a time of great changes in that realm. ATT UNIX was in the process of getting re-written as BSD. There was also much legal wrangling going on around the re-write, slowing down the process. According to Linus, "If 386BSD had been available when I started on Linux, Linux would probably never had happened."