It's because what filter really turns is an iterator. This iterator doesn't really do anything until you start to use it's results, in this case when you cast it to a list. people2 is this thing that's ready to filter the list of people, then when list is called on it, it iterates through the list of people and delivers the filtered result. Now that iterator is done, there's nothing left for it to iterate over, so when you call list on it a second time, there's nothing there.
Read this for some more details - Lazy evaluation python
