The reason behind Minecon was a very simple and somewhat a bit of a shallow thing. Aside from it happening on the day of Minecrafts official full version launch. Notch wanted it to be a bit more special.
The only real reason as to why Minecon happened was because Notch wanted to go on a stage infront of a crown and press a button (in this case it was a Minecraft switch) that officially releases the game. That's about it. Sure Minecraft is a pretty cool game and all. With it's sandbox style world and giving you the ability to create what ever the hell you want. The first thing that came to my mind when I first heard about Minecon was "lol". But hey, atleast someone got a marriage proposal out of it.
Doesn't Notch look like Al, owner of Al's Toy Barn, from Toy Story 2?