I'm willing to bet they couldn't have done that because of the mind-boggling array of 1990s UNIX standards Solaris seemed to support. By the mid 1990s it was clear that the (platform-independent) GNU command line tools were generally superior to what was on Solaris, but presumably they had to keep all the old fossils around to satisfy their long-standing supply / support contracts with industry, defence, and government.

That was my guess back then, and it's my guess now. Someone please correct me if I'm wrong and you know for certain why.