Grenadaresearch,
I think the answer to your question "Why are Onion Bottle so rare" is fairly obvious.? Just like anything significantly old, they become rare with time.? If you find the right area or wreck, you will find the Onion bottles.? In the local rivers here in SC we have had ship traffic dating back to the 1500's and heavy traffic from the early 1700's on.? While ships were anchored at port, they would toss the bottles overboard.? I recently dove the site of an old 1700's trading post.? Most of the veteran divers claimed the site was dove out, but I had never dove it before and wanted to see for myself.? I found a spot on the opposite shore where a ship must have been anchored and found bottles dating back to the 1770's.? What is so amazing is they were mixed in with modern Pepsi bottles.? I would find a Pepsi or some other modern bottle, and right next to it would be one 200 years older.? I litterally found 4 identical beer bottles from the 1700's within a 10 ft radius of each other.? This is also the same area where I found the 3 gallon ceramic jug (pictured in my Avatar) whch dates back to the early 1900's.? The area is rich with history and bottles from every era can be found all right together.? Its just a matter of finding the right spot.
They are there, it may just take time.? Also, Onion bottles being so old, they may be covered up by sand or coral growth.? Most of the older bottles I find, I have to dig for or pull on coral growth to see whats attached.? I will use a knife or screwdriver to poke into the sediment and pull myself along.? If I hear the distinct sound of my blade hitting glass, I dig!? In this area, the rivers will have shoots of coral plants growing from the bottom.? They are attached to something!? Pull it up and see what it is.? Usually a fossilised bone, Megoladon tooth, or a bottle.
Good Luck,
Robert in SC