Scratch

One of the most important features in BYOB is first-class lists, which means you can pass lists as arguments, nest them (inside other lists, to make advanced data-structures) and so on..
Imagine making a 'sort %list' block without first-class lists. How would you do that? You'll have to pass the name of a list to the block (using the drop-down) but - wait! How would you access it? of course you'll have to get the list as argument, but then it will confuse users between '%list as text' and '%list'. Bad idea.
I don't really think there's a reason not to do that. It's just making data-structures as binary-trees and dictionaries difficult and slow to implement, because you either have to make a couple of lists (which CLUTTERS up the interface) or invent some complicated math formula to access the right item when you need it.
It's also not that hard to understand that feature. I'm pretty sure even a 6 years old child could understand the idea behind having a list inside a list. So why not?