I think it has a lot to do with tradition. There's a lot of history behind it, and people often have a lot of traditions associated with something that dates back as long as it does (1861). Particularly when it's an event of such significance to a relatively new and growing nation.

It also happens to be the richest and most prestigious "two-mile" handicap in the world, and one of the richest turf races in the world as well.

Then you have people enjoying the excuse to dress a little fancier than normal (plenty of people suit/dress up around here just to go down to the local pub for the afternoon), the excuse to duck out of work for a bit and have a beer at the pub with mates before heading back with hopefully a few extra dollars in your pocket, and also the great atmosphere at the pub (or wherever) when the race kicks off; everyone screaming and shouting over their beers and late lunches. It's a bit of fun, and there's an incredible amount of local and international money and horse-industry talent all centered on one big race.