Is an "excuse" required? When did this happen?
To me, the obvious default is doing your own thing. *IF* something like the OGL lowers the bar far enough to create a better option, then cool.
The GSL is only mentioned because it is, in theory, a surrogate for the OGL. But it fails to meet the standard the OGL established. So, the bar lowering criteria is not met and we return to the default.
Asking Paizo for an excuse now is like asking Steve Jackson games for an excuse for why they were not supporting 2E in 1994.