It's not ignorance at all... but unless you have done it, you would not know.
Because of the way the human wrist and thumb bend. Look at how high the FB part is, it is above the Stradella bass section. Now, I know how thick the accordion is.. think around 50% thicker than your average accordion.
Now, to use the thumb, I would need to insert the left arm to near the elbow and make sure the bass strap was *way* looser (to the point you would have a hard time controlling the bellows).
I know how hard it is for the thumb to hit that part of the accordion because I would have to be double jointed to hit anyFB button on my Morino in a "playable manner".
When I play FB on my Morino, it is not the wrist or the area just above the wrist that the bellows strap touches, it is *literally* mid-forearm and the strap is loosened to near it's end (and my forearms are NOT very thick... lol) and by design its a lot bigger than your average bellows strap just for this reason.
Now, if I really stretch to the point of near painfulness, I can touch the first row of the Free Bass' 3 rows with the side of my thumb, not the tip, which is what would be needed... but thats it. Now, replace that with piano keys, and that exercise gets even harder.
An easier way is to visualize it yourself. Take your hand, and in the air, place it in ordinary bass playing position. Now, bend that wrist more than 90 degrees so the thumb points down and is able to hit an imaginary note.
Don't try too hard. If you are like me and your forearm is pressed against the accordion (position needed to push/pull the bellows), you will either break the wrist or the thumb before you reach a playing position with the thumb where you can hit it with the tip of the thumb (either that or you have no either unscrewed the bass strap or removed the hand entirely from the bass strap to lift the elbow to the same required high position that the right hand needs to be able to hit the right side keys).
Make it easier to understand now?
