Probably because that landing page doesn't tell me much about how it works. It's "limited to 500 characters"... for the sender? What happens if it's more than 500 characters or has an attachment? Or wait, is this a walled garden thing? I can't tell.

So I check the "Why shortmail" page (which 99% of visitors will not do), and I'm still not sure what happens if incoming e-mails are longer than 500 chars.