A random list of reasons why.
1. It's summer so it's hot. Running around like a crazy person is even harder in the heat.
2. The kids finish the school year. All the end of school things that happen in the Northern hemisphere in Jun happen here in December. Concerts, presentation days, dinners, end of year sports parties - all happen now.
3. Most Australians pack up for their annual vacation or as we call them holidays and leave the office on Dec 24. It's usually about January 7 before the country is back in full swing. That means everyone is trying to get everything done before they head off.
4. Christmas and the summer break mean that everyone wants to catch up with you in December. This is a lovely tradition but people you haven't seen since May simply must catch up before Christmas. We're a drinking nation so basically you eat and drink your way through a hot December.