I think the reason is they still want the past to push forward the future. And they want new characters and expansion of the current timeline to flow from the prime timeline. If they want to revisit things in the past or explore the multiverse with the new characters, they can do that. I just think they're trying expand things too far too fast. Maybe slow it down a bit and focus on fans getting to know a new generation of characters. There will be some overlap and some will pass the baton quickly while others might take a while, or not ever relinquish it.