... because it's not possible to load the whole thread at once, so the normal scroll bar will be jumping up and down as you scroll to load content on demand. And really, if you want to jump to a particular post in a long thread, the special scroll bar is easier than pages anyways because it's continuous.
Really, the scroll bar IS a fresh take on the page list, except it's actually useful for navigating the thread and doesn't force readers to deal with the useless concept of numbered pages when such arbitrary divisions are no longer the primary way of navigating the thread. Once you choose to not implement explicit pages (and accept that it's not possible to load the whole thread in one request), the thread scroller is a natural consequence.