Because continuity isn't the reason we tell stories. We tell stories to explore interesting ideas. It can be interesting to explore an evolving, interconnected continuity, but the continuity is a means to that end, not an end in itself. If someone has a great idea for a story that can't fit with that continuity, they shouldn't be forbidden to tell it.
There is always a way to make the idea, you might just need a diferent sctor of space or a different era, thats all.