Why Do You Need To Be “invited” To Pinterest?

My friend told me about it and it sounds cool so I went to go sign up. I had to apply for an invite. I got this e-mail:
Hi!
Thanks for joining the Pinterest waiting list. We’ll be sure to send you an invite soon.
In the meantime, you can follow us on Twitter. You can also explore a few pins.
We’re excited to get you pinning soon!
- Ben and the Pinterest Team
What is that? Why can’t I just sign up and join and start making my own sets?