The answer, pretty clearly, is that the public would have found the unitary Executive answer quite unappealing. We expect prosecutors to exercise independent, professional judgment, rather than to pursue a political agenda. We certainly don't want them pursuing a partisan agenda.
Thus one hopes (or at least I hope) that this latest episode comes to be regarded as the pendular reaction to the left's too-eager embrace of Justice Scalia's solo dissent in Morrison. He wrote there, in condemnation of the Independent Counsel Act:
An independent counsel is selected, and the scope of his or her authority prescribed, by a panel of judges. What if they are politically partisan, as judges have been known to be, and select a prosecutor antagonistic to the administration, or even to the particular individual who has been selected for this special treatment? There is no remedy for that, not even a political one. Judges, after all, have life tenure, and appointing a surefire enthusiastic prosecutor could hardly be considered an impeachable offense. So if there is anything wrong with the selection, there is effectively no one to blame. The independent counsel thus selected proceeds to assemble a staff. As I observed earlier, in the nature of things this has to be done by finding lawyers who are willing to lay aside their current careers for an indeterminate amount of time, to take on a job that has no prospect of permanence and little prospect for promotion. One thing is certain, however: it involves investigating and perhaps prosecuting a particular individual. Can one imagine a less equitable manner of fulfilling the executive responsibility to investigate and prosecute?A decade after Scalia wrote these prescient words, liberals saw in Ken Starr's investigation of President Clinton the vindication of Scalia's warning. And perhaps Scalia was right. Perhaps the old Independent Counsel Act did not provide adequate safeguards against partisan uses of prosecutorial power. Yet Scalia's solution--rely on politics itself--has also been known to fail. It was, after all, the abuses of Watergate that gave rise to the Independent Counsel Act in the first place. That's not to say that politics will necessarily fail now. Perhaps Congress will be able to hold the Justice Department to account. But if so, it will only be because of the fortuity that the firing of the Justice Department Eight occurred during the one quarter of the Bush Presidency when Congress happened to be controlled by the Democratic Party.