The search box provides suggestions, but it does this by sending everything you type to your search provider. The address box doesn't, because we feel it's bad for privacy to send every web address you type to a third party.
FWIW, it's an option in Chrome. Simply uncheck "Use a prediction service to help complete searches and URLs typed in the address bar" in the preferences.
But with Firefox, your search box can still make suggestions while not sending web addresses to a third-party, thereby allowing you to have your cake and eat it too.
Perhaps we could do the reverse? Provide search suggestions if the user explicitly tells us to by beginning their input with a special character. '?' perhaps.
Alternatively, although not intuitive, if the user enters focus with the search bar through a specific key-stroke: CTRL + L for a regular URL entry, and re-route CTRL + K to our web-bar, but with suggestions.