> It continues to amaze us; every time we look carefully we continue to find that Planet Nine explains something about the solar system that had long been a mystery
Why hasn't the existence of an additional planet been hypothesized before?
Not saying this is a bad strategy, just that in my view it's not much different from hypotheses for additional planets which have come before.
The problem is that we have a lot of missing information about the orbital mechanics of bodies that far and the discrepancies can be sometimes so small that they can hide or be "explained" by observation and calculations errors.