Lincoln didn't make anything a state, Congress does that. And Congress passed the enabling acts for three territories, Nebraska, Colorado, and Nevada, in March of 1864. If Lincoln was that worried about the results of the 1864 election then why didn't he 'create' the other two states as well?
The rich deposits of gold and high-quality silver ore turned Virginny Town to Virginia City, the most important settlement between Denver and San Francisco. They also prompted President Abe Lincoln to make Nevada a state in 1864. Although it did not contain enough people to constitutionally authorize statehood he needed the Comstock to secure enough electoral votes to win the 1864 election.
I don't think there is a restriction in the constitution about the number of residents required to make a state, though there may have been some guidelines developed in one of the Congresses along the way (e.g., guidelines for the number of people to be represented by each representative in Congress).
I think a more correct characterization of Nevada at the time would have been that it only had about 15,000 residents and that was really too small to shoulder the burdens of state government, taxation, etc.
15,000 is about the present population of the city of Great Bend, Kansas. Hey, want to be a senator?
Others, such as Charles Dana who was in the War Department under Lincoln, said that Lincoln pushed for Nevada statehood to get the 13th Amendment through the Senate because a nose count at the time showed that the votes of senators from one more "state" was needed.