It's not "why", it's "how"
First, let's remember that Trump appears to have LOST the popular vote by a slight margin (counting is not yet 100% complete). Given that a Presidential election depends on the summation of the results in the various States, Trump's win resulted from nothing more than a series of razor-thin margins, many of them so close as to be statistical dead heats. Reading anything more into it than that the country is exactly 50-50 split along ideological lines would be folly.
Finally, the fact is that he HASN'T won UNTIL the electors chosen to the Electoral College have their individual meetings in their respective States on December 19, cast their actual votes, the results are transmitted to the President of the Senate and certain other authorities by December 28, and a joint session of Congress meets January 6 to count the electoral votes and certify the results.
How many US citizens (let alone people in the world) actually understand this process?