The main difference is that SUA never was popular enough. Nor was it very useful to most people because POSIX compatibility doesn't mean much to most projects,

The main difference is that SUA never was popular enough. Nor was it very useful to most people because POSIX compatibility doesn't mean much to most projects, but Linux, OSX, or BSD compatibility does. Having a system no one uses that's not compatible enough with other real-world systems is kinda a good reason for discontinuing it.