We can only speculate. The objections to Miers are more likely to be based on qualifications and cronyism, while the objections to Brown would be based on her interpretation of Constitutional principle.
Whether or not GWB is avoiding a fight based on Constitutional principle, in favor of a fight based on a charge of personal favor -- well, we can only guess. It is fair to assume that this was a carefully made choice.
Maybe and maybe not; so, why not have picked someone obviously like a Scalia....as he promised during his campaign...to get strict Constitutionalists on the bench?