Unix wasn't "good enough". It was, essentially, the standard for communication. The reason it was developed and adopted is to be used for what today we'd call a router. The idea was that big "real" computers wouldn't need to have their own networking components, instead, they'd connect to small and "dumb" computers, who, in turn would all use the same system, subsequently, would be able to connect and communicate with each other.

Unix was developed to be the small and "dumb" computer's OS. But, because it instantly monopolized the communication protocols, it became impossible to replace. Only a system that was 100% compatible with Unix would be able to do so, and that would be just another Unix.

In retrospect, it's very strange that a lot of programmers think that Unix is a good OS, or try to learn from its design. It's kind of awful, even after a lot of increments that tried to remove the awfulness and polish some rough edges, it's still really bad. And it's also getting worse, as virtually all long-living software does because of bolted-on functionality that wasn't anticipated by the original design.

Especially when “better” is expensive, but “good enough” is free.