Why shouldn't everyone in the USA vote for the Governor of California and every other state?
Why don't we elect Senators nationally instead of state by state?
Why don't we elect Representatives nationally instead of state by state?
Basically, it all boils down to the same reason.
Suppose, just for an example, that 60% of the people in the US lived within the city limits of NYC, which had expanded to cover most of NY state. Should the residents of NYC get to elect the President all by themselves? If you had this mass of folks in "Greater NYC", who would the politicians pander to and provide porkfests for?
Everyday, I'm extremely happy the Founding Father's were so bloody smart. They were almost prescient; maybe there will be time travel, eh? Maybe from Greater LA, a distant also-ran to Greater NYC in the year 2450 all the way back to 1776.