Maine and Nebraska state laws are not "carve outs."
There is and was no one preferred method envisioned by the Founders.
Unable to agree on any particular method for selecting presidential electors, the Founding Fathers left the choice of method exclusively to the states in Article II, Section 1
“Each State shall appoint, in such Manner as the Legislature thereof may direct, a Number of Electors….”
The U.S. Supreme Court has repeatedly characterized the authority of the state legislatures over the manner of awarding their electoral votes as "plenary" and "exclusive."
States have used a variety of methods over the years. Just in Massachusetts for example:
● In 1789, Massachusetts had a two-step system in which the voters cast ballots indicating their preference for presidential elector by district, and the legislature chose from the top two vote-getters in each district (with the legislature choosing the state’s remaining two electors).
● In 1792, the voters were allowed to choose presidential electors in four multi-member regional districts (with the legislature choosing the state’s remaining two electors).
● In 1796, the voters elected presidential electors by congressional districts (with the legislature choosing only the state’s remaining two electors).
● In 1800, the legislature took back the power to pick all of the state’s presidential electors (entirely excluding the voters).
● In 1804, the voters were allowed to elect 17 presidential electors by district and two on a statewide basis.
● In 1808, the legislature decided to pick the electors itself.
● In 1812, the voters elected six presidential electors from one district, five electors from another district, four electors from another, three electors from each of two districts, and one elector from a sixth district.
● In 1816, Massachusetts again returned to state legislative choice.
● In 1820, the voters were allowed to elect 13 presidential electors by district and two on a statewide basis.
● Then, in 1824, Massachusetts adopted its 10th method of awarding electoral votes, namely the statewide winner-take-all rule that is in effect today.
● In 2010, Massachusetts enacted the National Popular Vote interstate compact. This change will go into effect when states possessing a majority of the electoral votes (270 out of 538) enact the same compact.
These changes were accomplished using the Constitution’s built-in method for changing the method of electing the President, namely section 1 of Article II. That constitutional provision gives Massachusetts (and all the other states) exclusive and plenary power to choose the manner of awarding their electoral votes.