I wondered that, too. They have 2 million current Gmail users, and I think it was to prevent them from being deluged by everyone on the planet wanting an account at the same time. However, like every google app I know, they always leave you wanting more or better features. For example, in Gmail, there's no button for delete! Just a dropdown box.
Half the folks I know use IMAP so that Gmail will filter spam for their desktop email client.