Because they don't want to send all your keystrokes in the URL to your search provider just so you can get autosuggest. So there is one bar that does autosuggest and a different bar where you can put things that your search provider should not see.
Obviously Chrome doesn't have that problem, since they _want_ your search provider to see all the URLs you visit.
If you personally don't, then you can combine them yourself in Firefox right away: just remove the search bar entirely via the normal UI customization mechanism, and use only the URL bar, which doesn't do search autocomplete.
Based on the diffs there, looks like the 55 feature can at least be turned off by setting the "browser.urlbar.suggest.searches" preference to false... (That preference doesn't affect the search bar.) Also looks like there's preferences UI for this (the "Show search suggestions in location bar results" checkbox).
I think there were talks about not presenting the search box for new profiles, too, but it looks like that hasn't happened yet.