For one thing, it’s more sensible in terms of inheritance. The fact that Stack extends Vector is really strange, in my view. Early in Java, inheritance was overused IMO - Properties being another example.
For me, the crucial word in the docs you quoted is consistent. Deque exposes a set of operations which is all about being able to fetch/add/remove items from the start or end of a collection, iterate etc - and that’s it. There’s deliberately no way to access an element by position, which Stack exposes because it’s a subclass of Vector.
Oh, and also Stack has no interface, so if you know you need Stack operations you end up committing to a specific concrete class, which isn’t usually a good idea.