The same way I answered in Mahalo.com /answers/how-can-you-become-famous-on-twitter.
This question is as similiar to "Why is Ashton Kutcher so famous in Twitter?"
First, be famous.
Second, be really really twitting - personal stuff, share cool links, retweet, ask your fans questions and interact.
"Kutcher didn’t stop at Twitter though. He dragged his campaign onto the likes of YouTube, Ustream Tv, MySpace and Facebook. (Over 1.2 million fans follow Kutcher on Facebook alone.)"
Eventually, you'll pull followers when they find you have something interesting to share.