Re: It's not "why", it's "how"
the US electoral college announcing Trump as President elect, but then choosing Clinton as actual President early next year.
In theory at least, there isn't anything to stop an Elector from casting his (or her), own vote with, or without the blessing of his (or her), Constituents. Other then possibly inciting some major butthurt on the nation.
But this part of the Electoral College is IMHO, a complete historical anachronism, that should probably just go. To put it in as simplistic terms as possible. Think of the Electoral Voters as Absentee Snail Mails, and the US Capital as the Polling Station. Which is exactly how it was designed to work some 200 years ago, before the advent of say the Telefon, Radio, 24h News, and the Internet.