Because there's an uptick in usage of proxies that play MitM in order to provide caching.
Before, when it was only sensitive sites using https, caching ignored them, but with "everybody" doing it, it becomes imperative to add decoding to the proxy efficiencies.
For my own proxy, I can put in exceptions for my bank or credit card and not lose much in security (only for sensitive sites I forget to exclude), but for larger proxies at companies and institutions, its unlikely they'll bother to custom-add sensitive sites for all of their employees/users. They'll likely just rely on access control to the proxy machine -- which will be fine for most sites, but is less secure than if ssl traffic had remained "reserved" for sensitive sites.
It's a classic example of "unintended consequences".