Why, unlike most of its states governments, does the U.S. Federal Government have a more unitary executive, rather than a plural executive?
The convention reasoned that a unitary executive was to be preferred.
Wikipedia, Unitary executive theory:
The unitary executive theory is a theory of US constitutional law holding that the US president possesses the power to control the entire executive branch. The doctrine is rooted in Article Two of the United States Constitution, which vests "the executive power" of the United States in the President.
Plural executives exist in several states where, in contrast to the federal government, executive officers such as lieutenant governor, attorney general, comptroller, secretary of state, and others, are elected independently of the state's governor. The Executive Branch of the Texan state government is a textbook example of this type of executive structure.
Adoption of constitutional provisions
The phrase "unitary executive" was discussed as early as the Philadelphia Convention in 1787, referring mainly to having a single individual fill the office of President, as proposed in the Virginia Plan. The alternative was to have several executives or an executive council, as proposed in the New Jersey Plan and as promoted by Elbridge Gerry, Edmund Randolph, and George Mason.
At the Pennsylvania ratifying convention in 1787, James Wilson emphasized the advantages of a single chief executive, including greater accountability, vigor, decisiveness, and responsibility:
[T]he executive authority is one. By this means we obtain very important advantages. We may discover from history, from reason, and from experience, the security which this furnishes. The executive power is better to be trusted when it has no screen. Sir, we have a responsibility in the person of our President; he cannot act improperly, and hide either his negligence or inattention; he cannot roll upon any other person the weight of his criminality; no appointment can take place without his nomination; and he is responsible for every nomination he makes. We secure vigor. We well know what numerous executives are. We know there is neither vigor, decision, nor responsibility, in them. Add to all this, that officer is placed high, and is possessed of power far from being contemptible; yet not a single privilege is annexed to his character; far from being above the laws, he is amenable to them in his private character as a citizen, and in his public character by impeachment.
In 1788, the letters of the Federal Farmer were published, generally considered among the most astute of Anti-Federalist writings. The pseudonymous Federal Farmer defended the proposed unitary executive, arguing that "a single man seems to be peculiarly well circumstanced to superintend the execution of laws with discernment and decision, with promptitude and uniformity."
Meanwhile, Federalists such as James Madison were emphasizing an additional advantage of a unitary executive. In Federalist No. 51, he wrote that an undivided executive would strengthen the ability of the executive to resist encroachments by the legislature: "As the weight of the legislative authority requires that it should be thus divided [into branches], the weakness of the executive may require, on the other hand, that it should be fortified."
In Federalist No. 70, Hamilton wrote (well, a lot of gibberish1; but he also reasons about "unity" as it relates to "unitary executive"):
This unity may be destroyed in two ways: either by vesting the power in two or more magistrates of equal dignity and authority; or by vesting it ostensibly in one man, subject, in whole or in part, to the control and co-operation of others, in the capacity of counsellors to him.
But quitting the dim light of historical research, attaching ourselves purely to the dictates of reason and good sense, we shall discover much greater cause to reject than to approve the idea of plurality in the Executive, under any modification whatever.
Wherever two or more persons are engaged in any common enterprise or pursuit, there is always danger of difference of opinion. If it be a public trust or office, in which they are clothed with equal dignity and authority, there is peculiar danger of personal emulation and even animosity. From either, and especially from all these causes, the most bitter dissensions are apt to spring. Whenever these happen, they lessen the respectability, weaken the authority, and distract the plans and operation of those whom they divide. If they should unfortunately assail the supreme executive magistracy of a country, consisting of a plurality of persons, they might impede or frustrate the most important measures of the government, in the most critical emergencies of the state. And what is still worse, they might split the community into the most violent and irreconcilable factions, adhering differently to the different individuals who composed the magistracy.
1 According to Merriam-Webster, energy was first used in 1783 in the sense "dynamic quality". Without knowing that archaic use of the word, Federalist 70 (written in 1788), in parts, makes little sense; particularly, since Hamilton is referring to the office and not the person holding that office.