Because then when an attacker stages a MITM attack against your bank's website, you get no notification except the absence of HTTPS UI, which no one will notice. Whereas at present, if your browser starts to connect to an https:// URL, there's no way for anyone to stage a MITM attack without triggering a warning message.
(Of course, if your browser starts to connect to the http:// URL, like because you just typed the domain name without protocol into the URL bar, they can still MITM it. This is what Strict-Transport-Security is meant to prevent. That also causes all cert errors to fatal, so you can't MITM a site using STS even if you could persuade the user to click through a warning.)