On my experience, people are usually really bad at communicating. It requires additional effort to imagine what information might be required for somebody else to follow your train of thought. Think of code documentation: What the original author percieves as obvious is cryptic to somebody else because context has been omitted for the sake of brevity. The author needs to invest a lot of time to actually dump his entire mindstate into documentation. Even if he does make an effort, not everybody is a good writer. If he doesn't care about others understanding his stuff (which also seems to be the case with Mochizuki) it gets even worse.
Also remember that Mochizuki basically worked alone on his stuff for 20 odd years. He built his own world to work in and it's just as hard for collegues to catch up to his level in that world as it is for e.g. non-programmers to understand programming (which takes years).