Because otherwise only New York, Texas and California matter, which the framers of the Constitution decided was a bad idea.
I actually like the principle of the electoral college, it reminds me that the US is in fact, a Union of States.
The real issue I see are the rampant gerrymandering of the congressional districts, which have a far greater skewing effect on the results than the electoral college (well, technically the same because the congressional districts are identical to the electoral college).