Why did winning just the northern states

guarantee that Lincoln would secure the majority

of electoral votes?

In United States election, before an election can be successfully declared won and finished, one of the candidate needs to win the higher number of the electoral votes irrespective of whether he or she won the popular votes.

Lincoln winning the northern states only guarantees that he would secure the majority is as a result of those northern states having the majority of the electoral college votes ascribed to them irrespective of the size and population of those states.

Hence the reason Lincoln won with a margin 212-21 in the electoral college votes.