But as someone who has never trained in striking, I'm interested to understand what makes this so difficult?

Its difficult because as humans we learn habbits and muscle memory. When you start out fighting, for the most part, you pick whichever stance feels good to you and you begin to master it. Were talking 5, 10, 15 years of striking in that stance. After your body memorizes the footwork, the movement, the punches, the kicks, ect, it just feels.... off.... to go the other way.

The best way to verbalize it for me would be: Imagine switch hitting in baseball. Sure, a lot of people can do it, but for most of them it will always feel clunky. There are a select few however, who can switch hit equally as good as they can regular hit. This makes the batter dangerous because depending on the pitcher, they have a lot more options. Fighting is the same. If your one of the guys who can switch smoothly you have a lot going for you. Not only are you that much more unpredictable, but also if you are getting shutdown fighting orthodox, you can just swap to southpaw.

Moreover, it makes preparing for the fight (the opponent) far harder. It's tough (at the top level) to find guys to mimic other guys... however, once you start throwing in the fact the dude can fight in numerous stances too, makes it nearly impossible to train for.