Georgia requires that its candidates, including slates of Presidential electors, be elected with 50% of the vote. If Trump did not get 50% of the vote, the race would go to a December runoff.
That begs the question: What would have happened if it came down to GA deciding the election in a runoff if Trump say, got 49.8% and only won with 272 EV?
Actually,
the runoff requirement for presidential electors in Georgia seems like it was abandoned in 1968 and has not been re-instated since. Bill Clinton and Bob Dole both won the state with less than 50% of the vote in 1992 and 1996, respectively.
To answer OP's question, Metro Atlanta vote tends to report before the rural "downstate" vote. This is in contrast with northern states like Wisconsin and Pennsylvania, where rural votes are counted relatively quickly and very Democratic inner-city precincts take the longest to report. With Clinton performing very well in Metro Atlanta this year, more raw votes were needed out of South Georgia to show that she wasn't going to win the state.
In fact, I believe if HRC had matched Obama's 2012 performance in Non-Atlanta Georgia, Trump would have won the state with <1%. Had she matched his 2008 performance, she would have won it.
The rural areas of South Georgia are notoriously slow-voting, and the large numbers of counties (159, second only to Texas) also means that it takes longer for votes to be counted.
