Here's the exact reason, according to the official website " The first Oktoberfest was held in the year 1810 in honor of the Bavarian Crown Prince Ludwig's marriage to Princess Therese von Sachsen-Hildburghausen. The festivities began on October 12, 1810 and ended on October 17th with a horse race. In the following years, the celebrations were repeated and, later, the festival was prolonged and moved forward into September.
"By moving the festivities up, it allowed for better weather conditions. Because the September nights were warmer, the visitors were able to enjoy the gardens outside the tents and the stroll over "die Wiesen" or the fields much longer without feeling chilly. Historically, the last Oktoberfest weekend was in October and this tradition continues into present times".
There you have it, the September weather in Germany is so good that they decided to extend a festival that's essentially all about drinking. Wunderbar!