It's the fact that states use winner-take-all apportionment of electors.
Unlike the electoral college, winner-take-all apportionment isn't mandated in the constitution.
You could have the electoral college and also distribute electors according to the share of popular vote each party received in that state.
A 2 party system is another one. Some founding fathers even wrote against it [1]
[1] https://ritholtz.com/2011/07/founding-fathers-beware-two-par...
> In forty-eight states and D.C., the winner of the plurality of the statewide vote receives all of that state's electors; in Maine and Nebraska, two electors are assigned in this manner and the remaining electors are allocated based on the plurality of votes in each congressional district.[1]
[1] https://en.wikipedia.org/wiki/United_States_Electoral_Colleg...
Also, giving a bit more representation to less populous areas may be a feature you don't want to get rid of quite so hastily.