ANSWER: This is an interesting question with quite a bit of history. Way back in the first iterations of d&d, druids weren't a class of their own. Instead, they were considered a cleric subclass. The design of old-school d&d meant that clerics couldn't easily use slashing weapons. The scimitar was given to druids to differentiate them from the average cleric. Thanks to ancient ireland and celtic mythology, sickles are generally considered the quintessential druid weapon. The decision was made to continue this theme, allowing the druid to use "Crescent-shaped swords" as well. Later on, this definition was refined to just "Scimitars", and druids have kept the proficiency ever since.