It's partially because you can sign in with custom domains. By putting your email address in first, Google can figure out what signin mechanism to use prior to you inputting your password (or not, if you don't need one).
For example, we use Google Apps at work with our custom domain, with an internal SSO server providing authentication services. You enter your email address, the Google page directs you to the internal SSO server, you get a token, take that back to Google, and you get logged in - no password required.
For example, we use Google Apps at work with our custom domain, with an internal SSO server providing authentication services. You enter your email address, the Google page directs you to the internal SSO server, you get a token, take that back to Google, and you get logged in - no password required.