Because it looks good.
The design of about.me is clean, functional and intuitive. This site is actually genius because one of the webs biggest eyesores to me, personally, is the endless amount of social networking sites people put in their signatures.
But until about.me what else was an artist to do? Now all of your social networking can be connected to your about.me profile and your signature can look much cleaner. I personally link my about.me profile to the text 'more..' in my signature.
Wishing you continued success!