Pure economics. Problem presented to the gunsmith and remedied in easiest, most profitable way. A common problem requires a simple solution. I don't know whether the customer actually asked for the gun to be shortened (not uncommon) or whether maybe the smith had jigs set up or other reasons that made that cut either convenient or necessary.
A perspective: guns were handed down from generation to generation. They were used and abused. Luckily a broken gun got returned to a competent gunsmith who mended it. This could have happened several times to one gun. They broke and were, by nature, biodegradable, though not as biodegradable as those who carried or later owned them. Other guns were abandoned, blown up, and cannibalized for parts. So, actually, if you find a really clean rifle today that has lots of originality, you are a lucky person, and it doesn't need to be fancy, just real.