In no particular order:

1. You don’t like free beer.
2. You don’t like talking to other developers about geeky things like which framework is best or which episode of FireFly is the worst.
3. If you already know everything there is no know about Flex and Actionscript and building for mobile and Phidgets and LiveCycle and Axiis, and Typography, and Greenthreading and Agile development, and Flash Analytics and Flex 4 and Enterprise Multi-user applications and Git and Flexible games and Cold Fusion 9 and . . . you get the point?
4. If you don’t need to network with your peers – which means you already have the best job in the world and make more money than you know what to do with.
5. If you couldn’t get anything of value from talking with team members on the Flex Framework team.
6. If you have no interest in more cool swag.
7. If you would rather sit at your desk and read the tweets from the 360|Flex attendees about what a great time they’re having than actually be there tweeting about the great time you’re having yourself.

I could go on and on. Comment below if you can think of other reasons not to go.