At its inception, the EC was a parliament solely elected to pick the president; as a result, not all the electors in a state elected the same person. This started changing in the 1800s; states that wanted to increase campaign time in their state moved toward the winner-takes-all method, which then every state had to move to if they wanted to matter equally.

If EC votes in a state were allocated according to popular vote, then the existing system would be a lot fairer. Proportional EC essentially reduces the advantage of campaigning in swing states, since many more states become swing states due to the relative ease of swinging a single EV vs. a whole state. And it manages to do so without completely screwing over smaller states.

I did some napkin math a while ago for this, but if this was the case, Bush v Gore would've been decisively for Gore, Bush v Kerry would have been as close as OTL Bush v Gore, and Trump's margin of victory shrinks to about 20 electoral votes from 77.