There might be other reasons, but a big one is that having search and address bar in one box means that normal URLs you type into the address bar will get sent to the search engine for autocomplete by default. Since that's a major privacy violation, they're kept separate.

I've removed the search box myself. If DNS doesn't return anything or if what you type looks like a search query, it does a search anyway, so the only thing I'm missing is search autocomplete (there's an addon if you really want that anyway: https://addons.mozilla.org/en-US/firefox/addon/instantfox/).