UNIX originally wasn't closed source anyway, and most people like nowadays, only cared about the freebeer parts.

GNU only took off when Sun created the split between user and developer UNIX SKUs, which other UNIX vendors were quick to follow as well.

Suddenly having access to UNIX no longer meant having development tools around, if the server wasn't to be used by developers themselves.

Thus installing GNU became a common workaround to pay for a UNIX developer license.