Because it doesn't require my instantaneous attention and I get to control when I reply.
Neither does Twitter. Nor will Google Wave.
The problem with email isn't its asynchronous nature and that's not what's being declared dead here. Everyone loves that part. The problem is how its oversimplified inbox/outbox/folder/reply-to-all format overcomplicates multi-person and multi-project conversations. Gmail made a lot of progress by ditching folders in favour of labels and powerful search, but the basic problems still remain. Google was right, a brand new open protocol is needed that doesn't try to remain compatible with IMAP, POP and SMTP.
Twitter, on the other hand, is an awesome option for stuff on the other end of the scale - very quick casual questions or comments. Because it removes the social overhead of initiating email contact -- makes it more like just a comment to someone on the street. Twitter makes it easy to make contact and interact with both friends and strangers on a very light level. I've asked questions of people on Twitter that I never would have if email was my only option.