kakejiku
Chumono
Hopefully, this thread will promote respectful discussion!
Now, some "rules" are definitional. Like the height of a Shohin is 8 inches, max. If the tree height above the pot is 9 inches, it's not a Shohin. Why 8 inches? Why not 7? Or 9? I have no clue.
A postulation or guess, is that 8 is considered a lucky number in Chinese and Japanese culture.
Although slightly less well-known, eight is also a lucky number. This is due to its shape – 八. Called suehirogari (末広がり), it's lucky because it widens at the bottom which reminds one of prosperity and growth. Seven is also lucky, but nine is considered unlucky because the pronunciation of the number nine also mimics the word suffering.